Symptoms
A user has a group of recipients for whom approval from the moderator is required before the user can send them email. For some messages that are approved and delivered, the moderator and the sender receive notices that state that the messages are not approved. This situation might cause the sender to believe that the moderator didn't approve those messages.
Resolution
To fix this issue, install Cumulative Update 13 or a later cumulative update for Exchange Server 2019.