For Windows Server 2019, additional safeguards have been added to help protect from misconfigurations. We have added logic to check to check if the share is going to DFS.
Microsoft does not support running the File Share Witness on a DFS share and did not support it in the past and we will not support it for the foreseeable future.
In a Windows 2016 4-node multi-site Cluster with two nodes at each site running SQL FCI, where each side utilizing a storage replication for the shared drives. This connection to the file share Witness, a part of the DFS share.