Sometimes after the upgrade from version 3.4.2 to the latest version 10.5.1 version you may notice that some S/MIME certificates/keys/users cannot be viewed either by searching under Consumers ->Users or specifically for the keyID under Keys -> Managed Keys, even though a new email sent to one of the missing email addresses is showing the email was successfully encrypted against a keyID within the email logs.
The number of unviewable users may vary.
Enable the Key Cache setting to cache keys going forward.