I think I got to the bottom of it! 
So replication was working all along. The issue appears to have been with how I was registering the namespace after upserting the clusters. I wasn’t aware that when registering a global namespace, you need to specify the list of associated clusters for the namespace using the --clusters argument.
When I register my test namespace like so, everything appears to work as expected, I see the namespace and workflow executions replicate:
tctl --address "localhost:7233" --namespace "workflow-engine-sandbox" namespace register --description "<description>" --retention "1" --global_namespace "true" --ac "clusterA" --clusters clusterA clusterB
Previously, I was registering the namespace without the --ac or --clusters flags. Further, the TCTL help output doesn’t really specify how to format the list of clusters for the --clusters flag which also threw me off.
I happened to stumble across the TCTL documentation for registering a namespace and saw these additional arguments which pointed me in the right direction. I think it would be helpful if the XDR replication documentation made a reference to these necessary flags.
Hopefully this helps someone else, thanks for help everyone!