Unraid (6.12.4) not supporting mounting NFS v4.x

Hello,

I've been trying for a while to mount a NFS drive on another server using NFS v4.2. And every time, it fallback to v4.0.
This is really strange since the NFS server configured by Unraid has no issue talking in the v4.2 protocol.

Example of mount command:

root@Tower:~# mount -vvvv dolma:/Backs-Movies /tmp/test/
mount.nfs: timeout set for Tue Sep 12 21:01:32 2023
mount.nfs: trying text-based options 'vers=4.2,addr=192.168.42.12,clientaddr=192.168.42.57'
mount.nfs: mount(2): Invalid argument
mount.nfs: trying text-based options 'vers=4,minorversion=1,addr=192.168.42.12,clientaddr=192.168.42.57'
mount.nfs: mount(2): Invalid argument
mount.nfs: trying text-based options 'vers=4,addr=192.168.42.12,clientaddr=192.168.42.57'
dolma:/Backs-Movies on /tmp/test type nfs (rw)

I also tried with another server that I just configured NFS on, completely clean with Raspbian Bookworms.

root@Tower:~# mount -vvvv vpn-mgt:/srv/test /tmp/test/
mount.nfs: timeout set for Tue Sep 12 21:14:18 2023
mount.nfs: trying text-based options 'vers=4.2,addr=192.168.42.8,clientaddr=192.168.42.57'
mount.nfs: mount(2): Invalid argument
mount.nfs: trying text-based options 'vers=4,minorversion=1,addr=192.168.42.8,clientaddr=192.168.42.57'
mount.nfs: mount(2): Invalid argument
mount.nfs: trying text-based options 'vers=4,addr=192.168.42.8,clientaddr=192.168.42.57'
vpn-mgt:/srv/test on /tmp/test type nfs (rw,noatime)

It looks like an oversight in either Unraid configuration of NFS Mount or missing kernel config or some issue in slackware ? It's hard to say what's the cause, since Unraid support v4.1 and v4.2 as a Server which lead me to think its configuration issue somewhere.

I've asked for support to confirm the issue before opening this ticket.