To reproduce:
1. Open attached project
2. Open "spritesheet" scene
3. Press play

Notice two game objects: mickeyAnim_Image and mickeyAnim_SpriteRenderer.

mickeyAnim_SpriteRenderer Game Object with:
Sprite Renderer Component
Animator Component
• Does not get masked by uGUI masks components

mickeyAnim_Image Game Object with:
Image Component
Animator Component
• Gets masked by uGUI masks
• Does not animate sprite sheets

Animated sprite sheets don't work correctly on the uGUI canvas with Mask (Script) component.