Hi C.Azrael - the problem is, that after pressing "shutdown" in Proxmox, the libreelec standby/shutdown menu
is shown on your Display - if you confirm the shutdown by clicking "shutdown" in the menu, it succeeds in Proxmox.
There is not much you can do about this.
The qemu-guest-agent package does not exist for libreelec but could have helped here - as it signals the shutdown command directly to the VM.
So just remember to turn off your VM through ssh with poweroff.
Of course, rebooting libreelec from proxmox does not work either and only shows the Power Options Menu.