sorry .... misinterpreted the thrust of your previous post ... the majority of my FT (not overly extensive) experience has been on performance not handling (other than basic work). The phenomenon to which you refer, I suggest, is either going to be a result of one or more of
(a) unsteady flow affecting the PEC depending on the manner in which the stall is approached
(b) compressibility if the aircraft concerned are of sufficiently high performance with attendent high stall speeds
(c) Re if the data be not reduced to SL