This evening I found the North Arm unlocked and not managing to relock; this followed an hours-long drop of the transmitted power. Looking at some signals I found that PR_F7_Y moved by over 200 um, despite the drift control loop engaged during the lock. I found the source of the problem in PR_F0, where the vertical coils are saturated and causing the vertical drift of the suspension.
The West arm is still locked, because the drift control is moving BS_MAR_TX to compensate the misalignment of the input beam by the PR.
I put the North Arm in DOWN and left the West arm locked.