TAU N1mode sent by UE being reverted

Hi Experts.

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?

It was switching on/off during the whole DT.

Could be provoked by battery temperature?

Yes, could be due to temperature or some dynamic enablement/disablement if N1 mode based on some optimizations?

Actually, the reason was failure in registering on IMS, so voice centric Ue went from 5G SA and stayed on Lte

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

By disabling capabilities to access 5GS > disable N1

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