Battery Usage from 13.15 to 13.21

Confirmed in my case:

So, looking at my initial battery ilfe tracking with 13.15 I was seeing, on average, 6%/day with no activity and roughly 9%/day with 50 minutes of GPS activity. Which, coming from a VA3,, is awesome in my opinion.

With 13.21, (Same settings, including power down/up after upgrade) I am getting 11%/day without activities and 16%+/day For the same level of GPS activity....Approaching VA3 battery performance

Have you tried the following?
1. power off the watch for at least 1-2 minutes.

2. reset the watch (holding LIGHT for at least 15s). Syncing the watch right before that is a good idea to not loose recent data. HRV status history data stays intact (my experience).

3. Reinstall the phone app (or just Clear storage and Clear cache is enough when on Android; no idea what to do on iphone). You will need to set up some things, as they do not persist in the watch - pair the watch again, enable permissions, add again custom Text and Call responses. That is what I remember, may be little bit more. 

I do 1+2 after every SW update and since the last two updates also 3. The need of setting up some of the things every time is worth the potential problems.

Garmin really should offer a configuration backup/export function to make it so this stuff can be automated and reliably ported in after a firmware upgrade or device reset.  Keep a copy of your favorite config on your phone, computer, or cloud and import as needed. Probably just focus on native Garmin settings and not 3rd party apps and watch faces at least initially. 

Or better yet, as part of the update, list of any setting changes and give you the option to leave the current settings or make the suggested changes. The only reason I knew about the new sleep mode and watch face was bc I read about it here...

No third party apps... Yet! This is a new(ish) watch for me and has almost everything I want in stock configuration.

Per my first post, number 1 on your list has been completed. I too perform soft resets after every FW/SW update. I haven't tried 2 yet, most likely will. Question about 3 though... What benefit is there to doing this? When I have a problem, I usually go through all of my settings, on the watch (bc some are not available on the app yet), and confirm or change back to what I had before.

Thanks!

I missed the information that you did 1 already, sorry. Good that you also do 2. Garmin should add this information into the update instructions.

Regarding 3:

- some other users reported here, that the bluetooth syncing got somehow stuck and after performing 3, the battery drain was gone. I cannot confirm, haven't experienced that. So far, I had experienced battery drain like 2 times and performing 1+2 always helped.

- I noticed, that after playing around with changing watch activity settings from phone app (Garmin Devices -> Forerunner 955 -> Activities & Apps), the phone app starts showing some application setting incorrectly, different from these set in the watch. Resetting the phone app fixes that. Knowing that fact, I wouldn't be surprised, if updating the watch firmware brought some more inconsistencies in the phone app. 

- I have performed 1+2+3 already 2 times (immediately after syncing) - after upgrading to fw 13.15 and to 12.27. Had not experienced data loss and/or battery drain since then. Setting everything back in the fresh phone app is for few minutes (getting back all the text responses is quite annoying though).

When you say "fresh phone app" , do you mean you did a " clear app data" in Android app settings for that app (assuming Android)? 

Yes. There are two options, Clear storage and Clear cache, I do both.