For now I'm going to disable the notifications from the default app and download another Calendar app. I never really liked the way the Galaxy calendar app reminder is full-screen, anyway.
I am having the same problem since upgrading my Galaxy s6 Edge+ on ATT from Android 6 (Marshmallow) to 7 (Nougat). I don't have any answers yet, but will let you know if I figure anything new.
Re: MM ATT update out now!!
Mine is telling me about the update. But, it's requiring 3080 mb of free space. Seems like double, right? Even though I freed up 6 GB it still won't start the update. I'm already on a wifi connection.