The override camera must be active every frame, only enabling it sporadically won't work, because the game will set it back (un-rotate) in next frame. What this means setting camera and expecting it to getting the same value from the game won't work. You need to drive it from the Unity end.
Maybe you are using mirv_cals on the csgo end, be sure to check that they are not active (try with a clean config).
Cinemachine's Virtual camera Z-axis doesnt apply rotation value to override camera, X and Y is working correctly https://www.youtube.com/watch?v=yg39KpfS5qc&feature=youtu.be