看了看你发布的PaintCode链接（虽然，在这里回答问题更容易，而不必去其他地方阅读文章）...
是的，当半径大于矩形尺寸的1/2的1/3时，UIBezierPath(roundedRect: ...)似乎仍然有问题。
要“手动”创建圆角矩形路径，我们可以这样做（使用该链接的矩形尺寸和圆角半径）：

let r: CGRect = CGRect(x: 0, y: 0, width: 150.0, height: 153.0)
let cornerRad: CGFloat = 50.0

// center points for the corner arcs
let ptCTR: CGPoint = CGPoint(x: r.maxX - cornerRad, y: r.minY + cornerRad)
let ptCBR: CGPoint = CGPoint(x: r.maxX - cornerRad, y: r.maxY - cornerRad)
let ptCBL: CGPoint = CGPoint(x: r.minX + cornerRad, y: r.maxY - cornerRad)
let ptCTL: CGPoint = CGPoint(x: r.minX + cornerRad, y: r.minY + cornerRad)
    
let bez: UIBezierPath = UIBezierPath()
    
// Top-Right corner
bez.addArc(withCenter: ptCTR, radius: cornerRad, startAngle: .pi * 1.5, endAngle: .pi * 0.0, clockwise: true)
    
// Bottom-Right corner
bez.addArc(withCenter: ptCBR, radius: cornerRad, startAngle: .pi * 0.0, endAngle: .pi * 0.5, clockwise: true)
    
// Bottom-Left corner
bez.addArc(withCenter: ptCBL, radius: cornerRad, startAngle: .pi * 0.5, endAngle: .pi * 1.0, clockwise: true)
    
// Top-Left corner
bez.addArc(withCenter: ptCTL, radius: cornerRad, startAngle: .pi * 1.0, endAngle: .pi * 1.5, clockwise: true)
    
// close the path
bez.close()