In our application we recenlty changed implementation where we handled nbiot/ltm switching manually to using lte preference mechanisim offered by modem fmw 1.3.x. In general preference works as expected. But during testing in an emulated network envoirnment, our device never switched back to "prefered" system after it was connect to fallback system.
Following was the test setup:
1. Set LTM as prefered network on modem.
2. Enable cell with nbiot.
3. After a while modem successfully connects to nbiot.
4. Enable basestation with ltm while nbiot network is still available.
5. At this point, both nbiot and ltm cells are available to the modem, but modem did not switch back to ltm atleast with 10 to 15 minutes.
Above tests were done with mfw 1.3.0. For system mode we use
AT%XSYSTEMMODE=1,1,0,1 //ltm and nbiot enabled, gnss disabled, ltm prefered.
