Huh? What sort of case? Why would you think a settlement agreement include anything about "tax liability," and why would you think the omission of such a clause would render the agreement "illegal"?
How was this party aggrieved? Did this person sign the settlement agreement? What does "the appeal accepted" mean? Does it mean that the appellate court reversed what the lower court did? Otherwise, there is no "acceptance" when it comes to first appeals.
Nothing you have written indicates any sort of constitutional issue whatsoever. Of course, your post is vague and largely devoid of relevant factual information, so it's impossible to have any certainty about this.
If you thought the settlement was unfair, why did you agree to it? Dissatisfaction with a settlement is not a valid basis for an appeal.
What public policy? How were you coerced or under duress?
Without any actual facts, there is no way anyone here could speak intelligently about this.