Rotating a UI Mask object more than a specific amount kills its content

For some reason, UI Mask fails when you rotate it, causing its content to not render at all.

Have I missed anything, or should I report a bug on this?

Repro steps: Open the example scene (Or create your own mask + object to mask), select the ScrollView object, rotate it on the Z axis, and its content disappears after around 30 degrees of rotation.
(Running beta 20)

This does not happen in Beta 17 nor Beta 18