Around 21:54 UTC, July 13th, the BNS range dropped to zero and the sensitivity update in vim-online stopped. However, the ITF stayed on lock, even though the spectra of hRec and DARM displayed a marked, stable increase in the broadband noise, as shown in Fig. 1. This drop was therefore different from previous sensitivity drops, which were temporary, lasting only a few tens of seconds.
The operator on phone confirmed that indeed the detector had not lost lock, and that upon a reset of the power stabilization loop the sensitivity had gone back to normal.
An inspection of the INJ signals shows that shortly before the sensitivity drop glitches are present in the PSTAB_PD1 signal (Fig. 2); one glitch is large enough to be seen as a glitch also in the PSTAB correction signal. Seconds later (Fig. 3) other glitches occur and apparently cause an unlock of the PSTAB loop, since the power stabilization signals saturate, shortly before 21:54:21 UTC.
In light of this episode, it is plausible that most of the sensitivity losses occurring today are due to similar issues with PSTAB. To be confirmed, by inspecting individual sensitivity loss events.