Might not be germain to the problem posed but I have noticed exactly the same thing with my Waco. I can only put it down to incipient stall, as with a couple extra notches of throttle trim, and slightly less 'back stick', the problem does not manifest. Could be, with our lack of airspeed instrumentation, the effect of 'apparent airspeed/ground speed' as the airfield comes into vision under the wheels means that we cannot accurately see the true airspeed, and might be just getting a wee bit slow in those final few seconds of flight.
Evan, WB #12.