My ISM's HRV indices keep dropping

Same here. Can’t post image but it’s in this thread: Recent Stress not being sync-ed to Training Readiness

I am wondering if ISM independently "assess" a new HRV reading for 7-day or it supposed to "remember" from previous Garmin Devices' HRV reading , in my case, Fenix 8 two weeks ago... Please don't tell me that each Garmin devices reflect different HRV value, I am confused.  Please Garmin Support to clarify on this thing.

The individual overnight averages should similar to what your device was reading. Mine are lower than what my Epix was reading, hence the drop. I wore my Epix again for one night and that reading was up in my baseline range again. Makes me question which device is right.

The ISM does independently assess the data, I noticed this with the 7 day average RHR, it wasn’t accurate until it had 7 nights of is own collected data (it didn’t use any historic data from my Epix) Looking in the Garmin Folder of the ISM you can see how much data it requires/keeps. HRV is 20 days. I factory reset mine just over a week ago so will see what it says in a couple of weeks.