I was also finding this worsened battery, and following AUB3 update my battery wasn't lasting a full day.
Having read quite a few posts on this subject, last night i decided to do a full factory reset of my phone... back to a brand new clean install. I have read a lot of info from others, and those that have the best battery life seem to have been very clean in their S21 Ultra setup.
By clean, I mean NOT using the sync app Smartswitch... and only installing relevant things such as contacts and calendar can be taken from Google account data.
It is a bit of a pain to go back to the very start, but it can be a good spring clean.
When I originally got my S21 ultra on 28th Jan, I did a bit of uninstalling my old apps that i didn't want, and then i synchronised using smartswitch. I synchronised contacts, calendar, messages, apps and everything that i felt was necessary for my new phone.
To do a clean install... I wrote a list first of the essential apps I needed back, and then prepared to do a google sync of my contacts and calendar.
I then performed a factory reset, and then let the phone go through it's own update process of android etc... which took a while. The phone now looks completely different with OneUI3.1 and without syncing my old settings and apps.
The phone response time is noticeably quicker... and it is currently going through the battery optimising again. I have installed all the apps I want again, all individually from the App store. This is a bit of a pain, but I have skipped a load of the un-needed previous apps... i.e. easyjet, Tui, tripadvisor (no-one is going anywhere 
), loads of old games i likely won't play again... and that kind of thing... I am only installing banking apps... music and TV apps, newer games, and the kind of things i need right now for work, finance, TV and music.
Actually, some of these apps that have been re-installed now look completely different and have a new updated UI look to them... than how they looked beforehand when I synchronised my apps.
I now think that some of these apps have caused some incompatibility issues with OneUI3.1 and Android 11.
Anyway, time will tell, but I think my battery looks massively better today, although it is still optimising again, so I will know more in a few days.
Today i unplugged my phone at 7:00 to start work... it is now 13:00 and I still have 83% battery. Things seem to have improved considerably. I haven't used much SOT today but the early signs look promising.
If you have the time and the inclination, then I would recommend a factory reset with the new software version and without synchronising old settings and apps from earlier generations of phone.
Kind Regards
Smiley 😀
