Group Folders : why advanced permissions rules apply to every folders?

I’m trying to understand how Group Folder works. I have one Group Folder and 2 subfolders:
β”œβ”€β”€ dir1
β”œβ”€β”€ dir2
My only requirement is that users in group can only read content. But if I add an advanced permissions rule to dir2 (allowing one user to write in folder dir2), the new rule apply to dir1 as well !! In other words, my user can write in both dir1 and dir2, is that the expected behavior ? We can’t add a specific rule in a sub folder ?