Folder Permissions Manager does not show delegates added to a mailbox using the Exchange admin center. Also changing delegates using Folder Permissions Manager, removes any delegates added to a mailbox using the Exchange admin center.
This is a known issue with Microsoft Exchange Web Services (EWS).
Folder Permissions Manager uses EWS to get and update delegates. When getting delegates, EWS does not return delegates added using the Exchange admin center. When applying changes to delegates, EWS removes any delegates added using the Exchange admin center.
To solve the problem, manage delegates using only Folder Permissions Manager. Do not manage delegates in the Exchange admin center (Exchange admin center > Mailboxes > User mailbox > Manage mailbox delegation > Send on behalf).
Folder Permissions Manager, version 8 and later.