In addition to a problem with rotated UIs, ClipsDescendants depends on the Size property of a UI instead of its AbsoluteSize. If the AbsoluteSize and Size are different (due to the usage of one of the UI constraints) things like this happen:
(With UIAspectRatioConstraint on)
This is happening because the Size internally is not changed by the UIAspectRatioConstraint, rather you’re supposed to use AbsoluteSize if you want to know the actual Size.
Removing the constraint will show what the Size property actually is.
(Without UIAspectRatioConstraint on)
This can be fixed by simply setting the Size to the AbsoluteSize (and of course doing some conversions in-between), but it would be more quality of life if the ClipsDescendants property depended on AbsoluteSize.