What could be the possible reason for UE to send TAU on 4G with N1mode = 0 during the drive test and then after a while on LTE/NSA revert to N1mode = 1?
From 23.501
A UE set to “voice centric” for 5GS shall always try to ensure that Voice service is possible. A voice centric
5GC capable and EPC capable UE unable to obtain voice service in 5GS shall not select a cell connected only to
5GC. By disabling capabilities to access 5GS, the UE re-selects to E-UTRAN connected to EPC first (if
available).
By disabling capabilities to access 5GS > disable N1