Probably the code for Bluetooth isn't developed by Garmin itself but only third party code integrated into the software. This make things hard to debug  especially since not all units are affected. I am wondering why Garmin don't ask to diagnose affected units... 

Another thing is, if Garmin manufactures at different locations or buys parts for the units (like other big companies do) at second sources as well, this could indicate a hardware problem as well. This would explain why some units have no problems at all. Second sources are important for companies for economic reasons but can lead to problems if parts are in another tolerance band as expected or are of poor manufacturing quality 

Android. Rev. C based on the box. Not sure what the difference between a C and D would be.

Did a 45 min run outside, and lost 3% (from 82% to 79%). That was 5 hours ago, and since then only 1% loss (currently at 78%).

My watches are always connected to my iPhone X which is always up to date, as is the GCM version. We need to be clear which smartphone and which OS version and which GCM version before such sweeping statements can be made. For instance, there appears to be repeated issues with Huawai phones. Is that Garmin, Huawai, or Google's (Android OS) fault?.

It's not all 945's then. Mine's a Rev A and doesn't have any extra battery drain that I'm aware of and yours is OK.  Mine connects to a Pixel 3, all updated. But as mentioned, I connect to Blue-tooth very seldom, just have little need to do. So no, didn't mean for it to be sweeping statement.

I think some of this is down to the terrible Android app. I lose the connection between the watch and phone regularly and force stopping the app and cycling the Bluetooth restore it. Maybe the watch wastes lots of power trying to reconnect when down?

If those seeing problems or not can also mention the phone platform?

I have zero issues with the "terrible" android app, no loss of connection (I have the phone icon on the watch face and never see a disconnect), no need to force stop or cycle bluetooth.

Android 9, Samsung Note 9.

After the update to 4.00 I followed Garmin-Blake's instructions: Pressed the upper left button for > 15 seconds and charged to 100%. Before, I uninstalled battery widget because I wasn't sure that it didn't drain the battery. Since then my battery usage is about the same as it was with  3.30. I just had a 40 minutes run with music + GPS + OHR and it took about 8% battery. About 10% for a whole day without activity. Had a way higher battery usage with 3.90. Galaxy S10e connected permanently via bluetooth.

could it be, that only watches affected, where Android 10 phones used?

is there someone who have probs with iOS or Android 9?