Quite a few posts indicate that the New Experience iOS client doesn’t currently support IPv6, here’s the first three that pop-up in a search:
https://forums.plex.tv/t/remote-access-over-ipv6-on-new-plex-ui
https://forums.plex.tv/t/lack-of-support-for-ipv6-servers
https://forums.plex.tv/t/ipv6-support-gone
We don’t know for sure, because I don’t believe any formal responses have been posted by the Plex team, not confirming that IPv6 has been removed either temporality or permanently or confirming if this is a bug and if/when it might be fixed. Unless I’ve missed a post?