Reproduction steps:
1. Open attached project
2. Open and play the "clippingbug" scene
3. The left star will spin and blink (masked out completely) when it touches parent with RectMask2D component (check Repro.gif)

Reproduction steps without project:
1. Create a new project
2. Create a gameObject that has a Rect Mask 2D component
3. Create an image and make it child of the object with the Rect Mask 2D
4. Move the child towards bottom/top border of the parent
5. Child gets masked out completely

Expected result: The left star shouldn't blink (masked out completely). (check Repro.gif)

Note: The bug occurs only when the child is near parents borders and the parent has a enabled Rect Mask 2D.