Hmm, my understanding was that FQDN means that anyone will resolve the domain to e.g. the same IP address?

It just means it’s the full format, similar to absolute vs relative paths on a filesystem. jellyfin.myexample.com is fully qualified (technically there should be a trailing dot but that’s rarely enforced these days) - doesn’t matter what it resolves to. jellyfin is not fully qualified - nor is jellyfin.myexample. This matters when you start talking about records in different zones - for example you could have an A record for jellyfin in mydomain.com.