To reproduce:
1. Open the project attached by the user
2. Open the scene "ReproScene"
3. Select "Hero" game object
4. Open animation window
5. Select animation "Repro_HiddenFrames_Original"
6. Select all the frames within the animation
7. delete selected frames
8. observe how some frames are not deleted

Expected: no frames are left after deletion
Actual: some frames are left after deletion

Reproduced on: 5.3.6p2, 5.4.0f3, 5.5.0a5.

Note: 5.2.5f1 freezes on step 7 of reproduction steps