Reproduction steps:
1. Open attached project
2. Open the "Scene" scene
3. Select "GameObject" in Hierarchy and change its x rotation
4. Notice that when changing the x rotation every 90 degrees, 180 degrees are added to y and z rotations (check Repro.gif)

Actual result: When changing the x rotation every 90 degrees, 180 degrees are added to y and z rotations.

Expected result: The x and y rotation should stay the same.

Note: When changing the x rotation to 90 degrees and applying changes to prefab, the x rotation value changes to 89.98019 degrees.

Regression introduced in: 5.3