Documentation its saying, "ok, you can encrypt the files, only pgp users with valid keys can read. But depending on shared folder permissions (windows) , anyone may be able to modify the files (although they are not able to read them)
I agree with you , if the folder is not encrypted , pgp should not block the access.
What happen if you access directly to the folder using \\server\folder
Doi you see any useful information in pgp logs (tools, view logs)