Based on some concern about various clipping possibilities in EndY, Travis S. removed irises in the EY Rx side on Aug 15 (see WP 7111; this says EX but was fixed to say EY in paper book). When Sudarshan evaluated calibration data after this (alog 38235), he found that Rx calibration values had returned to "non-clipping" values from prior to end of May.

I wanted to confirm that this was true over time, not just on the day of calibration, so I pulled up the last 6 months on Dataviewer; see results in the attached photo. It seems the Rx PD does flatten out over the 2 weeks since iris removal.

Rick, Travis, and I are currently under the assumption that this indicates clipping at the iris due to small apertures being aligned at high incidence angles. However, this is far from conclusive and will need to be monitored for several more weeks or months. This also does not provide any clues as to the cause of the monotonic drop in output voltage since June.