implementing K9 mail wear support - stacked notification issue

I'm currently implementing K9 issue 619 "add Andoid Wear Support".
You can find the code in branch issue-619_AndroidWearSupport.
Basic support to delete(if no confirmation on the phone is configured), archive and spam all messages  of a given notification is done and working well.

My currently issue is that I added stacked notification exactly as documented.
The problematic code starts in MessagingController:4941 .

My summary notifcation is only shown on the phone, not the watch. OK.
BUT: my stacked notifications (With setGroup(...) but no setGroupSummary(true) are ALSO shown on the phone. These are supposed to be only visible on the watch.
Every documentation, example and StackOverflow posting shows me that I'm doing everything right and that there should be no way for this to ever happen.

I guess I need an additional pair of eyes to look through at this code.