Do I have a DOA USW-Flex-2.5G-5

I'm overhauling my network with UI and bought a USW-Flex-2.5G-5 as part of it. For some reason I can't get this specific device to adopt. It gets a DHCP IP and shows as a client on the uplink switch but just doesn't show as available to adopt.

On the other hand, I have successfully adopted a Flex Mini (the regular one) using the same uplink port so I know it's not a port/comms issue. Controller is running in Docker and I do have L2 discovery set up. I have also tried Option 43 and DNS pointing.

Any thoughts before I submit a RMA?