Pretty sure this is MEP related. 1st thing I would do is double check which gateway my clients are connecting to - 1st or 2nd? With MEP, depending on the configuration, even though the client connection says GW A, it might in actual fact be connecting via GW B. Once you determined that start with normal traffic troubleshooting steps.
I've for example have had instances where the network behind the gateways was pushing all office mode return traffic to GW A, even if clients were connecting to GW B.
If you want to retain the ability for the clients to manually decide which gateway they connect to - it's discussed in this thread
Then there's also a thread which I started which discuss a couple of options around MEP. I'm still firmly of the opinion that the MEP configuration process can be made smoother and clearer😁