As suggested by Dan Brown, I checked if the HWS could see any ITM beam spots movement during the May 10th/11th temperature excursion alog 69525. I could see nothing.
The attached plots of ITMX and ITMY show (L to R) the two locked before temperature changes and the two locks during temperature change. ndscope attached. HWS ref at at 76W input power compared to show time is 5 minutes after. Lock during the change was 50 minutes long. Videos didn't show anything either.
The problem was that the HAM-ISIs were all drifting off in Yaw (RZ) slowly, but surely, for weeks -- see LHO:69934.