Steps to reproduce the issue:
1. Download and open attached project.
2. Open scene named "RectMaskIssue".
3. Select game object named "MaskA" and notice that Pivot X is set to 0.5.
4. Set width of "MaskA" to zero.
5. Notice that the above text "Masked Text A" is fully masked.
6. Select game object named "MaskB" and notice that Pivot X is set to 0.
7. Set width of "MaskB" to zero.

Actual result:
Below text "Masked Text A" is fully unmasked.

Expected result:
Below text "Masked Text A" should be fully masked.

Reproduced width:
5.2.5f1, 5.3.6p3, 5.4.0f3, 5.5.0a6.

With 5.1.5f1 both texts are fully unmasked.