EDIT: Coincidentally, I turned off prevent cross-site tracking, because I got sick of raindrop.io signing me out constantly. However, that seemed, I think, to have solved this issue. The only other thing I changed was to turn on the compact tabs view, but I don’t see how that possibly could have had any effect on this whatsoever. So I think maybe the cross-site tracking prevention thing is broken, or something along those lines
You’ve covered the basics. Sorry you’re still having issues. Another thing to check is if your using iCloud private relay. That can get funny at times.
Good luck!