Selected post categories not respected in Latest Post

Hi,
I have MailPoet 3 Newsletters with Latest Posts Content.
However MailPoet does not respect the selected post categories (setting: include) for the Latest Post function, but includes all new posts (regardless of category). This is a major issue for us.
I have had this problem a while but since I cannot find any solution or existing answer in support forums, I need to ask you for help.

What can cause this issue?
Have you (or anyone in this forum) encountered this issue before, and if so, what solved the issue?

Looking forward to any help solving this problem.

Best regards,
Donald