To reproduce:
1. Open attached project
2. Open "test" scene
3. Select "GameObject" game object
4. Open Animation window
5. Select "New Animation" animation from dropdown list
6. Try to move any animation event to the other frame

Note: animation has to have Transform property

Expected behavior: event is movable
Actual behavior: cannot move the event

Reproducible with: 5.3.0b2, 5.3.0b4, 5.3.0f2
Not reproducible with: 5.2.3f1, 5.1.3p3