If anyone else runs into this problem, looks like maybe the latest version in F-Droid is broken:

https://git.jami.net/savoirfairelinux/jami-client-android/-/...

(I'm not going to try an older version, since I quickly found bug reports that suggest a pattern of breakage, and I already have to use way too much comms software that casually breaks. I might give it another chance later.)