A while back I had this solved in the #lxd channel.
The issue with freebsd 13.1 is with some escoteric feature that was altered in somewhere early lxd5 (pv_passthrough?) .
Try shutting down the VM and adding migration.stateful: “true” to the instances config.
Nice to see others running the BSDs. Someday the lxd VM agent will wind up there hint hint