I have a new S1 here connected to my WiFi.  I'm running XCS from my desktop PC on the same subnet and it is able to discover and connect to the machine as expected.  I have an older Pixel 6 here that I recently migrated off of, and it able to run the Play Store XCS app, discover my S1, and connect without problem.

My primary phone (and the device which is actually activated) is a Pixel 9. A week or so ago I was able to connect with this phone as I would expect.  Since doing that, I've upgraded the device to Android 15.  Testing yesterday and today I find that the app can discover the S1 but cannot connect.  When you click on the discovered device, it displays “Connecting” and then immediately displays “Connection Failed”.  Repeated attempts yield the same result.  

I've deleted the app and reinstalled, same deal (discovers the machine without problem but cannot connect).  Rebooted the phone and machine, same problem.

Phone and the S1 are on the same WiFi SSID on the same VLAN, and there are no east/west firewalls internally that would be blocking any communication.