Definitely not all that much into mobile apps, but I do believe I have read that there might be problems with those UI elements, such as button and sliders etc. As they are not like a "normal" objects, but are form control elements or at least they used to be, but not sure if that's the case anymore. As you normally wouldn't be able to draw anything above such objects, but it doesn't seem to be the case anymore. But regardless I would assume it still have something to do with that anyway somehow.
You cant draw above them, just me not thinking straight
So they are still Form elements and will always go to front, which will cause problems now and then, that's why you normally don't use them for anything except testing purposes, I would say.