Why TAU Request occurs during 5G NR NSA attaching procedure?

Dear experts,

There is an EU, when it tries to attach to a 5G NSA band, it sends a TAU request message. That causes the attaching procedure incomplete. Assuming the device stays in one place, so we can ignore the mobility case.

According to eventhelix, the causes can be:

  1. the TIN indicates “P-TMSI” when the UE reselects to E-UTRAN (e.g. due to bearer configuration modifications performed on GERAN/UTRAN);
  2. a change of the UE Core Network Capability and/or UE Specific DRX Parameters information of the UE.

Is this a common issue? How do you troubleshoot it? What actually causes this problem? Why the device cannot attach to NR after the Tau procedure?