The error message is:
Cannot access the specified path or file on the server. Verify
that you have the necessary security privileges and that the path or file
exists.
As a test, we can create a new database, verify that it exists, detach it, and then immediately try to re-attach it, and we get the error.
We of course have verified that the Administrative User, the SQL Service Account, and the Computers all have full rights to this data directory.
We opened a case with Microsoft, and after 4 hours on the phone, were told that this appears to be a bug when using a UNC, presumably on a Scale-out-file-Server, for our data.
The workaround is to use a SQL command to attach the database instead.
Create
database OldData On
(Filename = '\\sqlserver-1\SQLServer-Data\SQLdata-1\Data\OldData.mdf'),(Filename ='\\sqlserver-1\SQLServer-Data\SQLdata-1\Logs\OldData_log.ldf')
for ATTACH