A couple of months ago, I posted on the Index Sleep Monitor forum with a detailed write up detailing how the ISM when compared to other Elevate V5 watches, records higher heart rate and lower HRV.
When inspecting the ISM's "settings.fit" file, Resting Heart Rate is set to 50 bpm. The ISM seems to rely on this RHR setting as the device's "true north" to correct for when there is poor signal.
I created python code to specifically target the settings.fit file and successfully changed the resting heart rate field to my true value. What I found is that, upon syncing, it defaults back to 50 bpm. It seems it might simply be a server-side issue, not the device itself.
I can confirm that FW 3.91 did not improve the heart rate detection/calculation issue. So, as of now, any heart rate data measured by the ISM will be high for users with a RHR of less than 50bpm. I cannot confirm how the algorithm changes the beat detection measurements for users with a higher RHR.
It is clear, at the very least, that there is an issue with Garmin Connect pushing incorrect RHR data to the ISM. The question is, if this sync issue is corrected, will it improve the RHR and HRV numbers to align them with other Elevate 5 devices?
(+) Update to the ISM and poor heart rate and HRV detection, FW 3.91 - Index Sleep Monitor - Health & Wellness - Garmin Forums
(+) Evidence that the Index Sleep Monitor produces bad heart rate data - Index Sleep Monitor - Health & Wellness - Garmin Forums