Apologies, marking this as solved for anyone else maybe having this issue. Seems it's an issue with the "ZFS Master" plugin and the latest releases of unraid. @Iker
Removing ZFS master allows the "main" tab to function properly again.
Noticed someone else now also mentioned this in the ZFS Master thread: https://forums.unraid.net/topic/122261-plugin-zfs-master/page/28/#findComment-1584670
Again, sorry for the noise but will leave this for anyone else maybe seeing the same thing. @Squid Plugin might need to be marked as broken for the time if it's no longer being supported, which seems it may be the case.