Good call on reading through the Filters link - unfortunately, working with filters can be complicated so I fully understand your confusion.
I believe there is a bug in the test-filters
command via the GUI (but possibly not the CLI), but I don’t know that it has ever been confirmed:
Are you backing up a UNC connection or just not sharing drive letters (C:) in your example top-level folders?
If it’s a UNC connection, then the problem is likely that the UNC starts with a slash so ends up including matching subfolders since they also start with a slash.