This is not something I have come across before .... when you have been looking at a banked stall, what were the conditions ie ..smooth air ?.. rate of change of bank ? ..rate of change of airspeed ? etc ... might give a clue to the answer ... I would have expected that, in a steady banked turn .. with the airspeed constrained to reduce slowly, the stall speed would be predictable. PEC ought not to be the problem .. Re is not varying ..and the mach number is too low to have any significance .... interesting ...