The question is extremely easy… I do not see much about this function in docs. Is estimateNewCameraMatrixForUndistortRectify (from fisheye) deterministic and so guarantees same output always for the given input? Not sure how camera estimation is performed in this case.
I would be amazed if it is not deterministic mathematically. Whether or not you get a great estimate every time is a different matter.
please add a MRE confirming your suspect, thank you