I have had this happening in my 2018 Q5 as well. Funny thing is that it did not happen for the months of Sept & Oct- I only noticed the Car Play taking over from the Radio since November, which if I recall was around when iOS11 came out. (though I note the OP posted in March - so not sure about that)
Would be great if folks with the Audi MMI system running an iPhone pre-iOS11 could comment on if they are having the problem. I have a strong suspicion that it was part of the Do Not Disturb / Car Play package that screwed this up. So now I have to choose between listening to the radio & charging my phone - Thanks Apple!