iOS 12 is currently still a beta and, as Apple note on its website here, that means that if you are using it your OS, "may contain errors or inaccuracies and may not function as well as commercially released software."
These inaccuracies can lead to issues with the way Forza and your OS interact and some users have noticed issues with their notifications from Forza.
We have let Apple know about these problems and they are working on fixing and tweaking iOS 12 and so we hope these issues will be resolved as Apple releases new versions of iOS 12.
If you have any further questions about this, you can get in contact with us through the app, as detailed in this article: https://help.forzafootball.com/general/how-do-i-contact-support
Or email me firstname.lastname@example.org