How to reproduce:

1. Open Unity and create a new project.
2. In the scene view create a new Cube GameObject.
3. In the Inspector window change the Transform Rotation position.
4. Open Animation window and create new Cube animation.
5. Select any other frame after 0 and change Cube rotation.
6. Go back to the 0th frame and observe how Rotation values are changed.

Expected behavior: The 0th keyframe for the rotation property should have the rotation values of the object before editing its rotation.
Actual behavior: The 0th keyframe for the rotation property is seemingly random and is neither the new rotation position (after editing) or rotation position before editing.

Reproducible: 5.4.0b21-25, 5.4.0f1,
Not reproducible: 5.3.5p8, 5.2.4f1, 5.4.0b20, 5.5.0a3