I have been using survey123 with external gnss rtk (chc i90).
And it worked perfectly, untill Android 12 update on my Samsung galaxy s21 5g.
The gps is paired successfully throw Bluetooth settings, but when I'm trying to setup the connection on the survey123, the gps can't be found.
I tested on other s21 devices with android 11 and it's working, updated it to Android 12, and the same problem.
I checked on field mapp app and it's working as well.
One possible cause may be the Android update cleared the NMEA settings. I would suggest downloading the native application for the GNSS receiver from the Play Store and ensure that NMEA strings are enabled.