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