Here you need to make some engineerining judgement. Is 19 faces a significant portion of your outlet? How strong is the backflow? Fluent will report the message when the velocity has even a tiny component opposite the outward pointing normal. For example, if there are 20k faces at the outlet and most have an outflow of 1 [m/s] and 19 have a minor backflow of -1e-3 [m/s], than your solution may be good. Also the area of interest should be far away from the exit.

And because you do have backflow, you need to go back and double check what you are specifing as the backflow conditions make sense in regards to what the rest of the solution is telling you.