I believe that is a misunderstanding of the rhythmic value that corresponds to the downbeat in question, and the tempo it is being played at. If it is an 8th note or smaller played at a fast, medium or even slow-medium tempo, no problem for an approach note to fall on a downbeat, especially if it resolves immediately. Remember, jazz is all about tension and release! On a slow ballad however, it isn’t such a good idea…
If you analyze solos by many of the great bebop players, you’ll find that this is the case. By the way, at the end of Module 8 I will start a series called “Dissecting the Masters”, through which I will demonstrate the above principle, among many other things!