Okay, will try and report
Okay, that seemed to do the trick.
I reset the machine-id using the commands you suggested, and then stopped/started the VM instances to force them to pick up a new IP address (just doing netplan apply did not seem to work).
Now, am able to correctly resolve all combinations … from containers to local and remote containers/VMs, and from VMs to local and remote containers/VMs.
Thank you both for jumping on this and spending a significant fraction of your morning.