I would try 2 things:
First, ensure that network port TCP 135 (RPC) is open in the host not receiving backlog (create an exception rule in Windows Firewall if needed: https://learn.microsoft.com/en-us/powershell/module/netsecurity/new-netfirewallrule?view=windowsserver2019-ps)
Second: open the DFS management console and make sure the links are enabled for that server
And, something that fixes in most cases, if everything else is alright: pull out the issue member from the Namespace, reboot and then add to the replication set. Once it is replicating successfully, add to the Namespace.
If this doesn't help at all, I would follow up by looking into DFRS Events for potential clues in the Errors or Warnings.
Hope this helps you narrowing it down,