Recently, not long after the battery life on my Instinct 2 Solar has been abysmal.

Same issue since the 13.21 update. Haven't changed any settings. I'm usually getting 3-3.5 weeks on a charge. After just one week says I only have 6 days of battery life.

I am also having a similar issue. Prior to the recent update would have great charge now 100% - 15% in a day without activities logged, Garmin support advised a factory reset which hasn't helped as charge is still dropping quick! Would suggest its a software issue!

I would tend to agree. I haven’t changed my usage in any significant manner, maybe 1 more activity a day, but no major settings changes, and suddenly I’m charging every day instead of every week or every other week. 

My battery is draining in a few hours,I wake up and it's dies overnight, full charge in the morning and it's dead again by the afternoon 

Same!! Very frustrating!! 

Update: I tracked percentage use yesterday. (only 3 activities 2 walking and 1 treadmill

11:30a: 99%
12p: 96%
1p: 92%
2p: 86%
3p: 80%
4p: 74%
5p: 64%
6p: 54%
7p: 45%
8p: 32%
9p: 19%
put on charger.  charged to 95%

8a: 16%

Pretty abysmal performance, and I haven't changed the way I use the watch at all.

can it be solar version specific bug? i have regular instinct 2, updated recently to 13.21 and was scared would get the same issue - but nothing. actually looks like it's better in terms of battery... (might be sth to do with solar charging or NFC/wallet?)

It very well could be.

Same issue after updating to 13.21. Anyway to revert to older FW versions?

Better to contact Garmin Support and work with them to find a solution to your problem.