Hi… I’m not with Fly.io, but I think this is a known phenomenon, rather than a platform bug.
I was able to get through to the password prompt…
$ psql "sslmode=require sslsni=1 host=spectral-health-db.fly.dev user=postgres"
Password for user postgres:
psql: error: connection to server at "spectral-health-db.fly.dev" (2a09:8280:1::82:c81c:0),
port 5432 failed: FATAL: password authentication failed for user "postgres"
(Older versions of psql don’t have the sslsni knob, unfortunately.)
Hope this helps!
Aside: If you want to connect via IPv4, it would also be prudent to double-check that you have a dedicated address. (And remove the one labeled shared.)