[USG Flex H] - Different device/same Mac Address after change interface to port

Hello everyone,
Today I've changed an interface for one port, disconnecting the old device and attaching a new one.

After this, when I've tried to connect the new device, it's not acquired an IP and under the DHCP list, I see the new IP for the new interface but the MAC Address was attached to the old device.

Example:
P8 (Interface LAN) —> Samsung TV with mac address AA:BB:CC:DD:EE:FF IP 192.168.1.100
———————————— Change configuration to ———————————-
P8 (UPS Interface) —> after connected an UPS —> mac address AA:BB:CC:DD:EE:FF IP 192.168.200.2 but …. the mac address of the UPS is different :)

So… no ping, no ip address on the device (instead for the firewall the IP is 192.168.200.2), no communication

At this point, I've disconnected the UPS, released the old IP from the DHCP table, reattached the UPS and now is connected successfully with different mac address (the real UPS mac) with 192.168.200.2 IP.

It's seems that also if you change the interface for a port, the port don't release the old mac associated to the port.

Probabily the problem is the lease time, but I don't want to "stress" the arp table; my solution can be a sort of deletion of the entry into the arp table if config for a port changing. Can be a valid solution?

Other people that had the same problem?