OSX Server File Sharing allowing more access than configured
You probably made a mistake and got the permutations wrong.

After creating a new share use the defaults and set the standard POSIX permissions for the group and everyone to "No Access". Then add the two users (here UserA and UserB) with the +-button.

Example: Shared-Folder1



In the Terminal it looks like this for all three folders:

host:~ admin$ sudo ls -lae /Volumes/Data/Shared-Folder1/
total 0
drwx------+  2 admin     staff  -  68 Oct 12 01:07 .
 0: user:_spotlight inherited allow list,search,file_inherit,directory_inherit
 1: user:usera allow list,add_file,search,add_subdirectory,delete_child,readattr,writeattr,readextattr,writeextattr,readsecurity,file_inherit,directory_inherit
 2: user:userb allow list,add_file,search,add_subdirectory,delete_child,readattr,writeattr,readextattr,writeextattr,readsecurity,file_inherit,directory_inherit
drwxrwxr-x  11 admin  staff  - 442 Oct 12 01:11 ..

host:~ admin$ sudo ls -lae /Volumes/Data/Shared-Folder2/
total 0
drwx------+  2 admin     staff  -  68 Oct 12 01:07 .
 0: user:_spotlight inherited allow list,search,file_inherit,directory_inherit
 1: user:userb allow list,add_file,search,add_subdirectory,delete_child,readattr,writeattr,readextattr,writeextattr,readsecurity,file_inherit,directory_inherit
 2: user:userc allow list,add_file,search,add_subdirectory,delete_child,readattr,writeattr,readextattr,writeextattr,readsecurity,file_inherit,directory_inherit
drwxrwxr-x  11 admin  staff  - 442 Oct 12 01:11 ..

host:~ admin$ sudo ls -lae /Volumes/Data/Shared-Folder3/
total 0
drwx------+  2 admin     staff  -  68 Oct 12 01:07 .
 0: user:_spotlight inherited allow list,search,file_inherit,directory_inherit
 1: user:usera allow list,add_file,search,add_subdirectory,delete_child,readattr,writeattr,readextattr,writeextattr,readsecurity,file_inherit,directory_inherit
 2: user:userc allow list,add_file,search,add_subdirectory,delete_child,readattr,writeattr,readextattr,writeextattr,readsecurity,file_inherit,directory_inherit
drwxrwxr-x  11 admin  staff  - 442 Oct 12 01:11 ..


If you try to access the share from a remote host as UserA it will look like this on a Mac:



and a Windows 10 host: