When a lease is first given out, Pool is set and shown correctly via on commit { set shared-networkname = "<pool name>"; } in shared-network scope, but after some time, likely on renew events, the shared-networkname is deleted from the lease, as is hostname.
The dhcp server logs show errors (these are for dhcpv6, but may be related, as dhcpv6 uses the same on commit):
Jun 24 04:58:23 vyos dhcpd: Renew message from fe80::xxxx port 546, transaction ID 0xDD257800 Jun 24 04:58:23 vyos dhcpd: Reply NA: address xxxx to client with duid 00:01:00:01:xxxx iaid = xxxx static Jun 24 04:58:23 vyos dhcpd: set shared-networkname: no scope
It may also be a bug in isc-dhcp.