Could you improve displaying static entries with more MAC addresses in admin UI (Settings > DHCP)? For ex. this is valid 04-pihole-static-dhcp.conf file entry:
Issue is that second MAC address is displayed in IP address field and Hostname field shows IP address. Hostname part is not shown at all. Inserting such entries through UI seems to work fine and I hope you keep it that way (not limiting to single MAC address).