I am not sure. Someone at Databricks once told me that mounts and Unity are not friends.

The easiest way to achieve this on file level is either:

• not using Unity and use AAD credential passtrough. then define the file access with ACLs on the data lake.
• forget about the file access and use dynamic views f.e. to create row level security.

Frankly using ACLs always gets on my nerves. Hard to maintain.