ZQ610 won't hold BT connection to Android

I have been trying to get the cordova plugin at cordova-plugin-zebra-printer 2.0.1 on npm - Libraries.io  to work with a ZQ610. I believe for this (and just about all cordova plugins that I could find for Zebra printers) your phone has to connect to the printer by Bluetooth (and not just on the same WLAN), right?

The problem is, if I pair the printer to my Android, after successful doing so the printer just seems to disappear from the phone connections after maybe a second (the bluetooth icon flashes on the LCD and then goes out). Interestingly enough, the Printer Setup app DOES connect to the printer (reportedly via Bluetooth)!

I have flashed the latest firmware (V85.20.15Z) and have tried explicitly switching off wifi as explained here - Zebra Printer does not maintain a Bluetooth connection but to no change. Printer Setup is OK, Android system BT connection disappears.

Have I got something completely wrong in my thought process? Thanks in advance for any help.