Unfortunately, as MANY very annoyed people continue to point out is that fetch is not working properly either and is on fetching when the app is running. For me, not only must the app be open, but The ipad must be awake. so if the ipad/iphone is asleep, generally i wont get mail. but sometimes.... not reliably and not every 15 minutes as it is set to do.
something unrelated or possibley related, i am also having a problem when all of my mail disappears,and has to re downloaded. Ill be in the midst of reading an email, and it will disappear in front of my eyes. i will then have to wait as 10,000 emails re-load. first the headers, then the messages. a slow process. this is happening a lot now, sometimes more than once a day, sometimes once a week. but it only started after the introduction of ios 11