A Playground to reproduce a cornerRadius anomality.

CornerRadiusPlayground.swift

//: A UIKit based Playground for presenting user interface
  
import UIKit
import PlaygroundSupport

final class Cell: UITableViewCell {
    
    private let borderLayer = CAShapeLayer()
    private let _backgroundView = UIView()

    override init(style: UITableViewCell.CellStyle, reuseIdentifier: String?) {
        super.init(style: style, reuseIdentifier: reuseIdentifier)
            
        // Add a blue background view.
        contentView.insertSubview(_backgroundView, at: 0)
        _backgroundView.backgroundColor = .blue
        _backgroundView.layer.cornerRadius = 28
        
        // Add the border layer.
        borderLayer.fillColor = nil
        borderLayer.strokeColor = UIColor.red.cgColor
        borderLayer.lineWidth = 2
        contentView.layer.addSublayer(borderLayer)
    }
            
    override func layoutSubviews() {
        super.layoutSubviews()
        
        _backgroundView.frame = contentView.bounds
        borderLayer.path = UIBezierPath(roundedRect: contentView.bounds, cornerRadius: _backgroundView.layer.cornerRadius).cgPath
    }
    
    required init?(coder: NSCoder) {
        super.init(coder: coder)
    }
}

class MyViewController : UITableViewController {
    let testView = UIView(frame: CGRect(x: 100, y: 100, width: 200, height: 100))

    override func loadView() {
        let tableView = UITableView()
        tableView.dataSource = self
        tableView.rowHeight = 84
        tableView.register(Cell.self, forCellReuseIdentifier: "Cell")
        tableView.separatorStyle = .none
        self.tableView = tableView
    }
    
    override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        1
    }
    
    override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        tableView.dequeueReusableCell(withIdentifier: "Cell", for: indexPath)
    }
    

}
// Present the view controller in the Live View window
PlaygroundPage.current.liveView = MyViewController()