Unfortunately, Safari 15 still has some problems here and there.
To get around i'd try
- checking for updates, maybe there's a new version where your problem is fixed
- use Migration Assistant to restore apps from Time Machine backup (if you did it beforehand)
- reinstall Safari (without reinstalling the OS) and see if it helps
- reinstall OS (your data will remain in place) - this will put back the Safari version your OS came with
Safari on 11.6.1 (Version 15.0 (16612.1.29.41.4, 16612)) is acting very odd for me, especially with facebook on busy groups. Seems to kill the tab and re-open it. I need to monitor what resources are getting eaten up.