What can be the reason of UE sending bye message with cause code DRB lost?

CORE
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1

Experts:

During 5G VoNR drive test we get call drop due to UE sending SIP bye message in good rf condition.

And cause code is DRB lost.

Can anyone put light what can be the reason of UE sending bye message with cause code DRB lost?

2

No RRC restab?

No UE at max tx power 23 dBm?

3

No RRC nor UE at max tx power 23 dBm.

4

Have you checked if there is any actual RRC Reconfig message or other message suggesting changes to DRB or IP filters?
Check the traces at IMS server and see what if any errors are observed before this message.
Low probability, check the error rate on VONR bearers, its possible bearers are up and running but the UE somehow getting higher error rate and for some reason thinks that bearer is dropped.