When a client sends DHCPDECLINE (it thinks the IP is already in use), lattice-dhcpd writes inventory quarantined and lease state quarantined.
TTL: LATTICE_QUARANTINE_TTL (default 15 minutes). lattice-worker SweepExpired returns the address to free.
Find them: Leases → state quarantined. Overview shows a warning while any remain.
Do not raise the TTL to hide clock bugs or a duplicate-IP war. Fix ARP / static leftovers / the other DHCP server. Do not set quarantine as a substitute for an exclude.
Counted against max_managed_ips while quarantined. See License.