Steps to reproduce:
1) Open the attached project 'AnimatorBug.zip'.
2) Open scene 'AnimatorBug.unity'.
3) Enter play mode.
4) Press the UI button 'Button'.
- The button, when pressed, instantiates a nested image prefab.
- A white box appears for a brief second and then disappears.

Notes: in 5.4, pressing the button sometimes results in normal behaviour - additional prefabs are created and the animator doesn't flicker, but if the button is pressed several times, the bug will still appear, and the button will become unpressable. In 5.3, pressing the button reproduces the bug on the first attempt and pressing the button again becomes unavailable.

Reproduced on:
5.4.0b20, 5.3.5f1, 5.3.4f1, 5.3.2f1

Did not reproduce on:
5.2.4f1