that was possible in the first place?

Are you an admin or something?

Yep, I'm a feddit.uk Admin and I was able to add an avatar and banner to !blackmagicfuckery@feddit.uk using the Photon frontend. Or I thought I did (I made a note of this at the time) but now my confidence in the sequence of events has been knocked. But if that wasn't his I did it, then I have no clue his I managed it.

I’d like to know what the behavior should be.

Well, I personally feel Admins should be able to edit the settings for communities they aren't Mods of (and I can't be made a Mod of a community because I have seniority). However, Lemmy's default is to not allow this (presumably as a holdover of the philosophy of Reddit where a sub was a Mods little fiefdom, at least until recently), so I presume that should be the behaviour of the frontends too.