Availability Group cluster memory issue after network issues. How to dump HADR Log Block Msg Pool?
I have noticed after every WAN issue where the WAN connection is flapping, and the off site nodes constantly disconnect and reconnect, the memory of the primary server gets consumed by the "HADR Log Block Msg Pool"


Yes, this is currently by design. It is expected that the network between the two sites can handle the traffic and is available. Since it seems that's not the case, SQL Server really isn't the problem here but is manifesting as an issue. If you're going to continue to work over an unreliable and possibly extremely high latency low bandwidth connection then I wouldn't use availability groups. In fact, I'm not sure what you'd want to use as nothing would have a solid and reliable connection which seems to be the root cause of the issue.


  Is there any way to dump this HADR Log Block Msg Pool?


Inside of SQL Server? No.


  Or stop it from growing so large in the first place?


Yes, fix the connectivity issue and it won't grow. If it's prolonged connectivity issues, then remove the remote replicas from the AG and it'll stop growing. Since there are two remote replicas, the data will be sent twice which could be exacerbating the issue as it might not have been taken into account for the infrastructure available when architected.


  Server MEM: 12GB


This is an awful small amount of server memory for 364 GB of databases + OS + Cluster + AG + all of the antivirus and agents installed.