Have you tried clearing the app cache? - yes
Does this also happen if you connect to our demo server (demo.jellyfin.org/stable)? - yes - same problem
Are you connecting to your server over IPv4 or IPv6? - ipv4
Via DNS or directly by address? - dns
Do you use https (via a reverse proxy perhaps)? - caddy
tested on 2 FireTV Sticks
Does this also happen if you connect to our demo server (demo.jellyfin.org/stable)? - yes - same problem
Are you connecting to your server over IPv4 or IPv6? - ipv4
Via DNS or directly by address? - dns
Do you use https (via a reverse proxy perhaps)? - caddy
tested on 2 FireTV Sticks