Read this whole thread. These people are really dumb, sorry they were being rude and not answering your question or simply not being helpful.
I was having the same problem bc I never take my phone off DND and tried something that seems to have temporarily worked.
On the banner, slide it over but don’t let go. Instead of clicking Clear, keep long sliding it with your thumbs alllll the way to the left. It’ll disappear on its own without you clicking. For the past hour of locking and unlocking my phone, it hasn’t popped up again, praise the Lord!!! I’m sure it’ll turn on again after restarting my phone but I rarely do that, so if that’s all I can get for now, I’ll be happy.
Hope that’s more helpful!!