VoLTE drops with Cause Value of Regular Deactivation

Hi Experts.

During VoLTE Drive Test we received some Drops with Cause Value of Regular Deactivation.

RF conditions are fine on that place.

And no ny abnormality on site as well.

Can someone guide me further explore of this Cause value?

Go check the other UE what he was doing.

And also post all the SIP messages of A and B party.

In the the party A: