Code exhibiting a sorting issue with MGLSymbolStyleLayer

IconSortViewController.swift

//
//  IconSortViewController.swift
//  IconOrdering
//
//  Created by John Newman on 11/18/19.
//  Copyright © 2019 Roadtrippers. All rights reserved.
//

import UIKit
import Mapbox

class IconSortViewController: UIViewController {
    
    private var map: MGLMapView?

    override func viewDidLoad() {
        super.viewDidLoad()
        let map = MGLMapView(frame: view.bounds)
        map.delegate = self
        map.debugMask = MGLMapDebugMaskOptions(rawValue: MGLMapDebugMaskOptions.collisionBoxesMask.rawValue)
        map.setCenter(CLLocationCoordinate2D(latitude: 38.1399, longitude: -112.1284), zoomLevel: 13, animated: false)
        view.addSubview(map)
        self.map = map
    }

    private func testShape() -> MGLShapeCollectionFeature {
        
        // Trying to draw the red icon on top.
        let redSort  = 0.99999898
        let blueSort = 0.99999896
        
        if redSort > blueSort {
            print("red sort > blue sort")
        }
        
        
        let featureDict: [String: Any] = ["type": "FeatureCollection",
                                          "features": [["type":"Feature",
                                                        "geometry": ["type":"Point",
                                                                     "coordinates":[-112.1284,38.1399]],
                                                        "properties":["text":"Red",
                                                                      "icon":"red_circle",
                                                                      "sort":redSort]],
                                                       ["type":"Feature",
                                                        "geometry": ["type":"Point",
                                                                     "coordinates":[-112.1274,38.1399]],
                                                        "properties":["text":"Blue",
                                                                      "icon":"blue_circle",
                                                                      "sort":blueSort]]]]
        
        let data = try! JSONSerialization.data(withJSONObject: featureDict, options: [])
        let shape = try! MGLShape(data: data, encoding: String.Encoding.utf8.rawValue) as! MGLShapeCollectionFeature
        
        return shape
    }
    
    private func drawShapes() {
        let source = MGLShapeSource(identifier: "my-source", shape: testShape(), options: nil)
        let layer = MGLSymbolStyleLayer(identifier: "my-layer", source: source)
        
        layer.iconAllowsOverlap = NSExpression(forConstantValue: false) // Flipping to true fixes the issue.
        layer.textAllowsOverlap = NSExpression(forConstantValue: false)
        layer.textOptional = NSExpression(forConstantValue: true)
        
        layer.text = NSExpression(forKeyPath: "text")
        layer.iconImageName = NSExpression(forKeyPath: "icon")
        layer.textOffset = NSExpression(forConstantValue: NSValue(cgVector: CGVector.init(dx: 2, dy: 0)))
        layer.symbolSortKey = symbolSortKeyExpression()
        
        map!.style!.addSource(source)
        map!.style!.addLayer(layer)
    }
    
    // When using floats, reversing the order doesn't change the outcome.
    private func symbolSortKeyExpression() -> NSExpression {
        return NSExpression(forKeyPath: "sort")
//        return NSExpression(forFunction: "multiply:by:", arguments: [NSExpression(forKeyPath: "sort"), NSExpression(forConstantValue: -1)])
    }
}

extension IconSortViewController : MGLMapViewDelegate {
    
    func mapView(_ mapView: MGLMapView, didFinishLoading style: MGLStyle) {
        let iconRect = CGRect(x: 0, y: 0, width: 30, height: 30)
        let renderer = UIGraphicsImageRenderer(size: iconRect.size)
        let red = renderer.image { (context) in
            UIColor.white.setFill()
            context.cgContext.fillEllipse(in: iconRect)
            UIColor.red.setFill()
            context.cgContext.fillEllipse(in: iconRect.insetBy(dx: 2, dy: 2))
        }
        let blue = renderer.image { (context) in
            UIColor.white.setFill()
            context.cgContext.fillEllipse(in: iconRect)
            UIColor.blue.setFill()
            context.cgContext.fillEllipse(in: iconRect.insetBy(dx: 2, dy: 2))
        }
        style.setImage(red, forName: "red_circle")
        style.setImage(blue, forName: "blue_circle")
        
        drawShapes()
    }
}