I don’t believe this is a new bug, just one that hasn’t been problematic enough to get written up. Still, it’s a confusing UX for any admin attempting to make a theme change on behalf of another user.
I noticed this area being odd, too, but hadn’t written it up yet. My notes were slightly different – Light & Dark pallette settings could be changed, though never seen from admin’s side:
When viewed by an admin account, a user’s Color Palette Light mode and Dark mode settings always appear as the admin’s own settings, regardless of the user’s settings.
Admin can successfully issue a change of Light mode and Dark mode settings for a user – but on refresh, the controls on the admin’s side always revert to show admin’s settings.
Admin cannot successfully issue a change to the user’s Theme.
Changes to Mode and Text Size from either end work fine, and the display of current settings sync as expected.
I also noticed this for “text size”. It depends on which browser I use to look at the user’s profile. I use a smaller text size only in Firefox, so I didn’t use “Make this the default text size on all my devices”. And when I check the profiles with Firefox, everyone seems to be using “smaller” just like me. In Chrome, I can see the real text size preferences, so I guess not using “Make this the default text size on all my devices” is the reason why I always see my own preference.
I wonder if that option also influences which theme preferences you see.