CGPath copy lineJoin and miterLimit has no apparent affect

I am offsetting a CGPath using copy(strokingWithWidth:lineCap:lineJoin:miterLimit:transform‌​:). The problem is the offset path introduces all kinds of jagged lines that seem to be the result of a miter join. Changing the miterLimit to 0 has no effect, and using a bevel line join also makes no difference.

In this image there is the original path (before applying strokingWithWidth), an offset path using miter join, and an offset path using bevel join. Why doesn't using bevel join have any affect?

Code using miter (Note that using CGLineJoin.round produces identical results):

let pathOffset = path.copy(strokingWithWidth: 4.0, 
                           lineCap: CGLineCap.butt,
                           lineJoin: CGLineJoin.miter,
                           miterLimit: 20.0)

context.saveGState()

context.setStrokeColor(UIColor.red.cgColor)
context.addPath(pathOffset)
context.strokePath()

context.restoreGState()

Code using bevel:

let pathOffset = path.copy(strokingWithWidth: 4.0, 
                           lineCap: CGLineCap.butt,
                           lineJoin: CGLineJoin.bevel,
                           miterLimit: 0.0)

context.saveGState()

context.setStrokeColor(UIColor.red.cgColor)
context.addPath(pathOffset)
context.strokePath()

context.restoreGState()