Not sure I saw this reported before. Today, I tried to align the y-arm. When I hit a certain angle in TMS, ETM or ITM, pitch or yaw, the lock would go bi-stable. When moving even further, it would stay at the lower value. However, the higher value could be regained by adjusting the PDH demodulator phase. As a matter of fact a phase change of ~20º can be used to switch between the states (see attached plot). Looking at the transmitted camera both modes look like 00.