To see whats going on, go to Ui issues with zindex - #26 by Ulxqra
Anyway, I put a frame with a ZIndex of 10, then a frame with a ZIndex of 5. The second frame went over the frame with the higher ZIndex. So I asked whats going on and they said change the behaviour to Sibling. I tried that and for some reason the roundify (Located inside the button) is going over the button with a higher ZIndex. Why?
You put the ImageLabel inside the TextButton, it will automatically go over it no matter what because its a child.
To fix this, and still have rounded UI, use the new UICorners. Roundify is a thing of the past at this point, and it will also fix your issue.