I would say that central point of contact is very small as compared to the size of dark circle observed.
You are correct that at the point of contact there should be no phase reversal as the same media (glass of plano-convex lens and flat glass plate) are in contact. Now what kind of size of this point of contact are you looking at.
Just beyond the point of contact there exits air gap between the curved surface of the lens and the top of glass plate, howsoever thin it may be.
See picture below where monochromatic light is shown from below. Which results in the central bright spot