I just recognized, that Topics and Posts from a restricted Forum can't be found on the Newsfeed, both, the one on the front and on the dashboard. I tryed it with multiple accounts and all them had the role assigned, which is also the one from the restricted forum. 

As soon as I remove the restriction, the content (new posts or new topic) is visibel on the feed.

I tryed it with different roles and users, but the result was always the same.

Is it meant by design, that restricted forums aren't visible on the newsfeed or is there something wrong?

Tanks for your help!