Thanks for the suggestion, I had seen it in other posts and tried it without success. I noticed that NoMachine is not in the Mint 22 software manager, but [removed] is, so I tried that instead, with a similar result. What seems to be happening is that the remote machine logs out when the monitor is disconnected (HDMI on a Nuc 8 running Mint 22) but not on the local machine (HDMI on a Nuc 6 running Mint 20). I have stopped there for now.