Hmm Are you wedded to that particular Mac address? If not, shut down the VM, delete the virtual Network card, then make a new virtual Network card. Copy paste the Mac of that new card into pfsense with the static mapping, and fire up the VM. See what happens.

If that doesn’t work, I remember something it was possible for proxmox to do some kind of routed Network system. To investigate that, delete all static mappings, fire up the VM, and just look at what Mac address it shows getting the DHCP lease. Is it the one that shows as being assigned to the VM?