Reproduction steps(5.3-5.4):
1) Open attached project and test scene.
2) Select cube in the scene view, notice it uses legacy Animation component.
3) Open Animation window, select rotation tool and add key frame by rotating cube.
4) Make sure Eulers Angles is selected under rotation(property) interpolation settings, there is no rotation in animation being recorded.(Also see attached gif.)

Workaround: Selecting Eulers Angles(Quaternion Approximation) under rotation interpolation settings in Animation window resolves the issue.

Reproduced with: 5.4.0a6, 5.3.0f4, 5.3.0f1, 5.3.0b1.
Did not reproduce with: 5.2.4f1, 5.2.3f1.