Steps to reproduce the issue:
1. Open attached project.
2. Open scene named "Scene".
3. In the "Hierarchy" window select rotated image named "Image".
4. Try moving "Image" horizontally in the "Scene" window.
5. Notice that the rotated image is clipped before it is completely outside of the mask.

Expected result:
The rotated image should be clipped when it is completely outside of the mask.

Reproduced with:
5.2.5f1, 5.3.5f1, 5.4.0f3, 5.5.0a6.

Cannot reproduce with:
5.1.5f1 (because this feature was not supported).

Note:
It seems like RectMask2d detects that X position of corner (of the rotated image) is lower than X position of mask's corner and clips the image. Vertically it works fine.
GIF attached.