Hi,
In this scenario , I think we need to make sure if this traffic is even reaching the ASA device Standby Interface or not ?
I think we might need to verify the issue by using a capture on the ASA standby device interface and see the issue.
Also , at the time of issue , collect the show tech from the Standby Unit , ping from the Standby unit to the Client IP etc.
Thanks and Regards,
Vibhor Amrodia