Thank you for the reply and suggestions.
Indeed, I previously look at those links and OpenCV source code, but before trying the USACParams option I wanted to have a better understanding of the behaviour of the default function. I can confirm that the functions have a seed that makes the results reproducible across multiple runs on the same data and using the same parameters.
As per your suggestion, setting a random state in the USACParams for each run makes the function behave again in a non-deterministic way, as one should expect.
Therefore, I would recommend to always use USACParams as argument of findFundamentalMat() and avoid using the default functions.
I also updated the code on my GitHub repo.