ThomasKenyon

Only one, I won't be on-site until Monday. Thanks.

It has always been on port 16 and always connected to a Unifi-AC-Lite. 

Thanks for your response, yes the problem still exists on reconnection. (This has happened three times now always the same port / device). (After the first time, I switched off 802.3az, but this doesn't make a difference).

If you are using multiple VLANs on the same port, you should probably set the VLANs up in the switch and set those ports to Tag Egress Packets for that VLAN.