Lockloss ~19:30 UTC was due to rapid rise (about 10 min into 50W) of new ITMY PI at 18136 Hz. It's now become MODE24 and was damped the following lock with the PLL scheme.
First saw mode in the live broadband monitor (found on PI medm screen under the StripTool button). I identified the arm by looking at SUS-ETMX/Y_PI_DOWNCONV_DC2_SIG_OUT_DQ spectrum at time right before lockloss. I identified test mass by alternating matrix elements to send damping signal to ETMY (with no result) then ITMY. Screenshot of PLL, broadband monitor, and StripTools during successful damping are attached.
Another new PI at ETMX 17771 Hz, now MODE7.
Belongs to the Xarm though test mass is unknown so far. Rang up about 2 hours into 50W lock (five orders of mag above noise floor in OMC DCPD) and rang down on it's own before I noticed it. I set up damping scheme in MODE7 and its been added to StripTool.