Naturally one of those CAS/HT servers also happened to be the File Share Witness for my two-member DAG. Whoops!
Fortunately we can resolve the problem by specifying a new FSW for the DAGm which I will demonstrate here
I’m going to use another member server within the site as my FSW, which allows me to demonstrate a related problem.
Set-DatabaseAvailabilityGroup dag-<DAG Name> -WitnessServer <Witness Server name> -WitnessDirectory C:\FSW
Please Note: If you are configuring to Non Exchange System, you should add the “Exchange Trusted Subsystem” security group