Jim P. on Nostr: Almost certainly the upstream router doing that or you have another device on the WAN ...

Almost certainly the upstream router doing that or you have another device on the WAN segment acting as a DHCP server.

My cable modem does that, for example, when it loses upstream sync. There is "Reject leases from" option on the interface settings to ignore DHCP leases from a specific server. Enter in the address of whatever DHCP server handed your router that address and that will stop it from being picked up in the future.