@susansnook Although I see what you’re saying, that sounds ambiguous enough to check with an attorney (if OP can afford it).
FMLA doesn’t just say that you can’t be fired. It also says that you can’t be penalized for taking it. So that includes performance reviews, performance based things, being demoted, etc.
So it sounds ambiguous, at minimum, as to whether withholding a bonus counts as a penalty. If they said, for example, “You obviously didn’t perform well because you took maternity leave”, that’s an illegal penalty!
If they pro-rate everyone’s bonus based on their paid hours, that may be different. But then, do they penalize you for taking paid vacation, for example? Will a male employee be penalized for a sabbatical?
It may be worth checking with an attorney at the 9to5 website that I linked above (if OP can afford it). I think 9to5 might do pro-bono work for an initial consultation, but not sure. Good luck!