Steps to reproduce:

1. Open attached project
2. Open scene "2D RectMask Repro"
3. In hierarchy, expand Canvas and select Button
4. Notice that it has rect mask component attached
5. In scene view, notice that it masks part of the button's text
6. Disable 2D Rect Mask and notice that text is still masked

Additional steps:
7. Save the scene and reload it
8. Notice that text is unmasked

Workaround: Reload scene after making changes to rect mask

Note: object stays masked even if 2DRectMask is deleted

Not reproducible: 5.3.3f1, 5.4.0b6
Regressions introduced in: 5.3.3p1 and 5.4.0b7
Reproduced with: 5.3.4f1, 5.4.0b10