What can be a reason for QCI5 Acc dip?
Vendor is Ericsson.
What do stats say?
Which failure rate increased?
Is this one cell, one site, one cluster, one city or one network?
This is in random sites from particular date.
Any specific counter suggested to check?
I would say you have specific faulty UEs that cannot register on IMS, most likely after a software upgrade.
Put some trace on such a cell and you will understand what it is.