Thanks for the reply. I just found the problem. I had a column which I used to order the months. After removing that, the view worked as it should have. I did not display the column but just used it as a sort field. If I had displayed it, I likely would have seen the months categorized in the column "> 30 days".
Feedback response number WEBBAYSN7Y created by Clem Padin on 05/15/2018