13.3 was produced for Core users that needed to update Jails at the time. I think it is EOL for that too? Forum post showing user updated a Jail to 13.4 TrueNAS CORE 13.3-RELEASE is Now Available - #214 by Davvo
FreeBSD page shows 13.4 as the lowest value in the current FreeBSD chains. I don’t use the FreeBSD Jails feature so I don’t know if it has fallen off the versions you can update like 13.0 did.
I think Core 13.3U1.2 is already EOL and will not get any more update. Link to docs and last release notice