I have noticed that one time. One area of the limb seemed stiff at just past brace height, then bent as it should through the rest of the draw. It did remind me of the oil-can effect, because, compared to the rest of the limb, it seemed to sort of go over-center and then bend.. That bow became a shooter and I don't recall whether the oi-can went away or stayed.
Steve may be right, about the changing string angle.
Jim Davis