Steps to reproduce:
1. Open attached Unity project
2. Enter Play mode
3. Open "Hugo" prefab in Prefab mode (observe starting prefab component values)
4. Enable Animator component in Inspector (notice that animated prefab component values start changing)
5. Exit Play mode

Expected results:
Animation stops playing and prefab values return to default after Play mode
Actual results:
Animation continues to play in Prefab mode and animated values are do not return to what they were before Play mode

Reproduced in: 2019.1.0a3, 2018.3.0b4, 2018.3.0a6

Note: Not reproducible before the Improved Prefabs update