Otherwise healthy DC failing DFS-R, pointing to DC that no longer exists

Today i had a DC that was otherwise healthy, but reporting error 4612 and 5012 in the DFS Replication log, specifically:

The DFS Replication service failed to communicate with partner <decommissioned DC name> for replication group Domain System Volume. The partner did not recognize the connection or the replication group configuration.

My first port of call was to open ADSIEdit.msc and check

CN=Topology,CN=Domain System Volume,CN=DFSR-GlobalSettings,CN=System,DC=domain,DC=com,DC=au

but the dead server was not in there.

After some googling, i found a reference to

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\DFSR\Parameters\SysVols\Seeding SysVols\DomainName\Parent Computer

and sure enough - this was referencing the now decommissioned DC - no idea how it happened. The new/old DC's were both online together for over a month - it should not have been still seeding.... but obviously something went wrong.

Updated the name to a DC that existed, restarted the DFS-R service, waited about 15 seconds - all is now good.