Workaround for Xamarin CollectionView + Refresh View + Header Issue (https://github.com/xamarin/Xamarin.Forms/issues/8282)

CustomRefreshViewRenderer.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using MyProject.iOS.Renderers;
using UIKit;
using Xamarin.Forms;
using Xamarin.Forms.Platform.iOS;
using Xamarin.Forms.PlatformConfiguration.iOSSpecific;
using VisualElement = Xamarin.Forms.VisualElement;

[assembly: ExportRenderer(typeof(RefreshView), typeof(CustomRefreshViewRenderer))]
namespace MyProject.iOS.Renderers
{
    public static class Extensions
    {
        internal static T FindParentOfType<T>(this VisualElement element)
        {
            var navPage = element.GetParentsPath()
                                        .OfType<T>()
                                        .FirstOrDefault();
            return navPage;
        }

        internal static IEnumerable<Element> GetParentsPath(this VisualElement self)
        {
            Element current = self;

            while (!Extensions.IsApplicationOrNull(current.RealParent))
            {
                current = current.RealParent;
                yield return current;
            }
        }

        public static bool IsApplicationOrNull(Element element)
        {
            return element == null || element is Application;
        }
    }

    public class CustomRefreshViewRenderer : ViewRenderer<RefreshView, UIView>, IEffectControlProvider
    {
        bool _isDisposed;
        bool _isRefreshing;
        bool _usingLargeTitles;
        nfloat _originalY;
        nfloat _refreshControlHeight;
        UIView _refreshControlParent;
        UIRefreshControl _refreshControl;

        bool IsiOs10OrNewer
        {
            get
            {
                return UIDevice.CurrentDevice.CheckSystemVersion(10, 0);
            }
        }

        public bool IsRefreshing
        {
            get { return _isRefreshing; }
            set
            {
                _isRefreshing = value;

                if (Element != null && Element.IsRefreshing != _isRefreshing)
                    Element.SetValueFromRenderer(RefreshView.IsRefreshingProperty, _isRefreshing);


                if (_isRefreshing != _refreshControl.Refreshing)
                {
                    if (_isRefreshing)
                        _refreshControl.BeginRefreshing();
                    else
                        _refreshControl.EndRefreshing();

                    TryOffsetRefresh(this, IsRefreshing);
                }
            }
        }

        protected override void OnElementChanged(ElementChangedEventArgs<RefreshView> e)
        {
            base.OnElementChanged(e);

            if (e.OldElement != null || Element == null)
                return;

            if (e.NewElement != null)
            {
                if (Control == null)
                {
                    if (IsiOs10OrNewer)
                    {
                        var parentNav = e.NewElement.FindParentOfType<Xamarin.Forms.NavigationPage>();
                        _usingLargeTitles = parentNav != null && parentNav.OnThisPlatform().PrefersLargeTitles();
                    }

                    _refreshControl = new UIRefreshControl();
                    _refreshControl.ValueChanged += OnRefresh;
                    _refreshControlParent = this;
                }
            }

            UpdateColors();
            UpdateIsRefreshing();
            UpdateIsEnabled();
        }

        protected override void OnElementPropertyChanged(object sender, PropertyChangedEventArgs e)
        {
            base.OnElementPropertyChanged(sender, e);

            if (e.PropertyName == VisualElement.IsEnabledProperty.PropertyName)
                UpdateIsEnabled();
            else if (e.PropertyName == RefreshView.IsRefreshingProperty.PropertyName)
                UpdateIsRefreshing();
            else if ((e.PropertyName == RefreshView.RefreshColorProperty.PropertyName) ||
                (e.PropertyName == VisualElement.BackgroundColorProperty.PropertyName))
                UpdateColors();
        }

        protected override void SetBackgroundColor(Color color)
        {
            if (_refreshControl == null)
                return;

            _refreshControl.BackgroundColor = color != Color.Default ? color.ToUIColor() : null;
        }

        protected override void Dispose(bool disposing)
        {
            if (_isDisposed)
                return;

            if (disposing && Control != null)
            {
                _refreshControl.ValueChanged -= OnRefresh;
                _refreshControl.Dispose();
                _refreshControl = null;
                _refreshControlParent = null;
            }

            _isDisposed = true;
            base.Dispose(disposing);
        }

        bool TryOffsetRefresh(UIView view, bool refreshing)
        {
            if (view is UIScrollView scrollView)
            {
                if (scrollView.ContentOffset.Y < 0)
                    return true;

                if (refreshing)
                    scrollView.SetContentOffset(new CoreGraphics.CGPoint(0, _originalY - _refreshControlHeight), true);
                else
                    scrollView.SetContentOffset(new CoreGraphics.CGPoint(0, _originalY), true);

                return true;
            }

            if (view is WkWebViewRenderer)
            {
                return true;
            }

            if (view.Subviews == null)
                return false;

            for (int i = 0; i < view.Subviews.Length; i++)
            {
                var control = view.Subviews[i];
                if (TryOffsetRefresh(control, refreshing))
                    return true;
            }

            return false;
        }

        bool TryInsertRefresh(UIView view, int index = 0)
        {
            _refreshControlParent = view;

            if (view is UIScrollView scrollView)
            {
                if (CanUseRefreshControlProperty())
                    scrollView.RefreshControl = _refreshControl;
                else
                    scrollView.InsertSubview(_refreshControl, index);

                scrollView.AlwaysBounceVertical = true;

                _originalY = scrollView.ContentOffset.Y;
                _refreshControlHeight = _refreshControl.Frame.Size.Height;

                //todo: check if fixed
                //added to fix https://github.com/xamarin/Xamarin.Forms/issues/8282
                _refreshControl.Bounds = new CoreGraphics.CGRect(new CoreGraphics.CGPoint(0, -scrollView.ContentOffset.Y), _refreshControl.Bounds.Size);

                return true;
            }

            if (view is WkWebViewRenderer webView)
            {
                webView.ScrollView.InsertSubview(_refreshControl, index);
                return true;
            }

            if (view.Subviews == null)
                return false;

            for (int i = 0; i < view.Subviews.Length; i++)
            {
                var control = view.Subviews[i];
                if (TryInsertRefresh(control, i))
                    return true;
            }

            return false;
        }

        void UpdateColors()
        {
            if (Element == null || _refreshControl == null)
                return;

            if (Element.RefreshColor != Color.Default)
                _refreshControl.TintColor = Element.RefreshColor.ToUIColor();

            SetBackgroundColor(Element.BackgroundColor);
        }

        void UpdateIsRefreshing()
        {
            IsRefreshing = Element.IsRefreshing;
        }

        void UpdateIsEnabled()
        {
            if (Element.IsEnabled)
                TryInsertRefresh(_refreshControlParent);
            else
            {
                if (_refreshControl.Superview != null)
                    _refreshControl.RemoveFromSuperview();
            }
        }

        bool CanUseRefreshControlProperty()
        {
            return IsiOs10OrNewer && !_usingLargeTitles;
        }

        void OnRefresh(object sender, EventArgs e)
        {
            IsRefreshing = true;
        }

        void IEffectControlProvider.RegisterEffect(Effect effect)
        {
            VisualElementRenderer<VisualElement>.RegisterEffect(effect, this, NativeView);
        }
    }
}