To reproduce:
1. Open attached project from RectMaskIssue.zip.
2. Load "testScene" and enter play mode.
3. Look at the Game window.

To reproduce from the scratch:
1. In a new project create a panel (from UI).
2. Add RectMask2D component to that panel.
3. Add animation which would increase the width of the panel.
4. Create 2 (or more) child images, for that panel.
5. Place 1 of them inside the panel bounds, and other outside (but in the bounds of panel when it's width is increased by the animation).
6. Play scene.

Expected result: Parts of images are rendered, when they are inside the bounds of an animated panel.
Actual result: Images which were not inside panel's bounds at the start, are not rendered.

Reproducible with: Unity 5.4.0f3, 5.4.1f1, 5.5.0b9
Not reproducible with: Unity 5.3.1f1, 5.3.6f1