Cross-posting from the linked thread for others with FP2 Open SafetyNet issues:
So it does look to be like @Olli_GT said before me:
It’s not microG that doesn’t work with SafetyNet, it’s something to do with either the way µG is installed on FP Open, or something different with the ROM itself. In my post I suggested reaching out to µG devs for help, I think that’s the best course of action if one wants to get SafetyNet on FP Open.