Repro steps:
1. Open "Unity-invisible spewns" project
2. Make sure "TestScene" is loaded
3. Enter play mode
4. In the game scene drag and drop red and/or blue squares on the right white canvas
5. Observe that some squares disappear randomly and then reappear after you drop more squares
Note: GIF Attached. Squares also reappear when disabling and enabling "UI Canvas" in the hierarchy.

Actual behavior: Instantiated prefabs with image component once dragged and dropped to Rect Mask 2D disappear randomly
Expected behavior: Instantiated prefabs with image component once dragged and dropped to Rect Mask 2D do not disappear

Reproduced on: 5.4.0f3, 5.5.0a6
Did not reproduce on: 5.3.6p3