Hi Euripides, thanks for thinking about this.
This is correct. The ubuntu mini-pc uses its highest-priority wifi connection to connect to my home wifi network. It obtains an ip address, can access the outside world and I can use VNC Viewer to remote-control it by connecting to BEELINK::5901. No change here, this has always worked.
This is only partly correct. When I turn off my home wifi, ubuntu correctly creates a wifi hotspot according to its second-priority connection setting. On my windows laptop (and also on a windows desktop pc) I see the hotspot and can connect to it. The windows box says "Connected", but does not obtain an ip address from ubuntu. On the ubuntu machine, arp -a shows no connected devices.

Sorry for being unclear. There is no 3rd situation like this, only #1 and #2.