No E-Mail Notifications will be sent as soon as a owned written Follow-up Action is activated

 Hello,

We have following problem and don't have any idea to fix it.

A colleague has written a "Follow-up Action" for the section "Save Work Item" it is called "Default Subscriber". This Follow-up action only adds defined user to a work item as subscriber if a defined category was used.
But if the Follow-up action is activated we have the problem that the Owner and Creator of a Work Item don't get an E-Mail Notification. If you're only Subscriber of the Work item an E-Mail Notification will be created.

At the moment we have no idea why no E-Mail Notifications will be scheduled.
Our Follow-up action only adds Subscriber to a work item and save it.

Maybe someone has a similar problem and already fixed it and can give us a hint?

Thanks & Regards
Tobias