I think things may have gotten into a non-intuitive state that is (as far as I know) primarily intended for Fly Postgres machines: the app exists and has a machine, but there are no “releases” in the strictest sense.
Maybe try the following, first, to confirm:
$ fly app releases -a floating-islands-game-server
$ fly m status -d -a floating-islands-game-server
$ fly logs -a floating-islands-game-server
For the second one, you’re looking for fly_release_id and fly_release_version as metadata keys.
(And the third will typically help resolve doubts about why it’s crashing, 
.)
If the hypothesis does hold up, then you might be able to use --update-only to introduce a release without the creation of any machines.
(But I’m just guessing now.)