You use Folder Permissions Manager to apply permissions on public folders on Exchange 2016 CU8, but the permissions are not actually being applied.
This is a known issue with Exchange 2016 CU8.
Permissions are correctly applied by Folder Permissions Manager using Exchange Web Services (EWS), but Exchange 2016 CU8 does not actually apply the permissions.
Upgrade Exchange 2016 to Exchange 2016 CU9 or newer.
Folder Permissions Manager, version 8 and later.