In addition to the accepted solution and as a quick recap for myself and other future readers:
- double-check that the dhcp negotiation actually works, especially if you have a firewall
- the client may cache an existing lease and prefer using it until expiration, rather than accept your new static lease candidate
- Android (SLAAC-only hosts) need a "config domain" entry with the MAC-derived address: you must manually create it, since it is not automatically created from the existing "config dhcp" entry for IPV4 static leases
Thanks to all who contributed.