SQL Server Distributed Availability Group databases not syncing after a server reboot
Please note, this is not a definitive answer but it's the best answer after chatting with Taryn.


  However, the primary was showing a very different story. It was reporting that the separate AG was syncing without any issues but the DAGs were in a Not Synchronzing / Not Healthy state


If the individual databases and AGs underlying the distributed ag say they healthy and synchronizing, there is a good chance this is just a hiccup in the DMVs and/or SSMS dashboards. Since there was nothing in the errorlog to suggest the replica didn't connect or was in a disconnected state. 

Unfortunately since the issue has resolved, it's hard to say exactly what it was... but in the future if this occurs for someone:


Check sys.dm_hadr_database_replica_states on all clusters looking for anything that isn't healthy. If all shows healthy, it's possible the DMV hasn't updated yet
If it's unhealthy check the errorlog/DMVs for connectivity issues (such as not being able to connect to the forwarder/global primary)
Dan's answer mentions issues that could arise from database startup - though in this case the instance can't be read so that most likely wasn't an issue but could be in your case
If the database is readable, smoke test with a dummy table/insert or ...
Extended event session using the DEBUG channel items sqlserver.hadr_dump_log_block or sqlserver.hadr_apply_log_block to see if the secondary is actually receiving/applying the log blocks or ...
Perfmon object SQLServer:Database Replica\Log Bytes Received/sec


If you're receiving data on that secondary but the distributed ag still shows not synchronizing or not healthy then I'd let it go for a bit to see if the DMV values change since it's obviously receiving and processing log blocks.

If, however, it isn't then we'll need to investigate further which is out of scope of the answer.