Its under families policy https://play.google.com/about/families/ads-monetization/ "Banner ads showing multiple offers"
I believe Admob (by default) swaps to a new advert periodically if the banner is open a long time. I'm not sure why that would be default if it's not allowed on the Play Store though.
Its true that ad gets refreshed and its allowed, whats not allowed is multiple offers within a single banner ad.
My only other guess is that one of the adverts you are showing is against the T & C's, which would be rather frustrating.
That's my guess too, but i am also constantly looking for my options to display ads in admob to find a setting that could may be disable such ads.
When you get a violation you can always respond to it, and you will get further information from Google, what they claim is wrong and how it should be. You will have the respond link in the email you receive from them
I have responded to the email, my game was rejected several times, i responded and they just send me typical guidelines and best practices.
My app got reviewed again and again, many times violated the same policy and got removed temporarily, i updated again and by luck the banner may have shown a single ad a the time they reviewed my app and so it was approved, but then in next update the same violation happened.