Notification mail with MS Teams integrated planner

Hey guys, 

we are using MS Teams with the Planner-APP integrated. Since a few weeks we have an issue for guests to recieve any notification mails. When we use planner directly without MS Teams we have the same issue. There is acutally no fix for this? It worked pretty well ~6 month ago. 

We checked all settings and activated every notifications in planner for the User himself and for the Plan.

Any suggestions? 😞