FWIW: I found another clue to this issue. On a DIFFERENT phone, I experienced the same issue. I created a new session and set it up to use a Digirig TNC. When I was done, none of my previously created sessions would work.
I opened one that I had for my MobilinkD TNC3, and the TNC configuration was missing my TNC. I tried to put it back and the ONLY available option was TCP/IP (no Bluetooth, no BLE, etc). After trying to edit several different sessions on the phone and everything else I could think of, including accessing the TNC3 from the MobilinkD app, I restarted the phone. After the restart, I could see all of the previous TNC options again and I could add the Bluetooth TNC3 back to the configuration.
Ken KB7WHO