Remember also that any closed-form approximation to reality will have some anomalies where the algorithm will "blow up" and give unexpected results - even if the model is right, if one of the intermediate terms in the calculation is very near to zero or the maximum value for the numeric type, the "finite" calculation may be far different from the real-world result. [Look up chaos theory and the theory behind numerical analysis of differential equations...]. In this case, it does sound more like there are some mass items that OpenRocket accounts for differently than RockSim. In any event - if you can't fix the computer model in OpenRocket for this particular vehicle, use something different. I think WinRASP is still out there too...as is the original G. Harry Stine RASP - I have a "translation" of it that's written in C, which I still use occasionally to confirm OpenRocket or RockSim results.
Mark J
Old-fart mathematician