Deployment target is set to 14.0 for all parts of my app (Main app, Widget extension, Watch extension and this one as well). So I doubt it could cause this issue.

The line with " if let bestAttemptContent = bestAttemptContent {" is in the template of Xcode and does not come from Google at all. That is how Xcode creates this service extension if added. And the print yields nothing because it is never getting called. I receive the push on my device but this extension won't get called for some reason..

Thanks for the reply. The line you mentioned is from the template that Xcode generates if you create such an extension. So if this is the problem, Apple included a not working template which would be very bad. The debugger is also never hit if I place breakpoints in it, even if they are on line one of the function. And yes, FCM is the Firebase Cloud Messaging which seems to work (I get push notifications at least but I don't know if something prevents it from including the mutable content flag; tried "mutable-content" as from Apples documentations as well as "mutable_content" from the FCM documentation).