Very very late into this thread, but, I had this issue after getting my 2011 B8 S4 rear bumper repainted - I pointed this out to the Audi approved repair centre manager, he just pressed it in hard and of course it came back out again, so as I just wanted to get away from there I thought that I'd sort that out "later"!
That was maybe 10 years ago, I've still got that car and that point, on the rear bumper is still not flush, but, I've had a couple of attempts to sort it out and have come to the conclusion that there is too much surface leveller or paint at a certain point on the inner side of the bumper and that is making it impossible for any built in clips in that area all engaging correctly - and, while I was working on it, I found that the other side of the bumper - ie the LHS could not be disengaged from its plastic mounting plate! So I stopped there, maybe next time, if there is one, I'll remove the mountings for the rear LHS bumper mounting plate and get the rear bumper off completely, then address this second issue and resolve the first one on the RHS.
Did you ever get your rear bumper mounted correctly/perfectly, and if so, what needed to get done to it?