I'm not sure what's going on, but the list of leases in the DHCP table is less than the list of IPs in the ARP table.

There is no other DHCP server on the network, and yet somehow devices have been allocated IPs not showing in the DHCP table.

Confirmed the ARP table is correct with ARP-Scan.  I'd not noticed this behaviour before upgrading the firmware.

@John,

Just a thought, but do you have any static assignments, or bound ip's to MAC?

John.

I did find the problem. There were two MAC addresses with the same IP address!
I think one was a misbehaving client. Having identified the problem I left all alone and it sorted itself out after a few hours.