[Jim S]

Think about the intent of the rule. The purpose is to give A the same option if B commits a live ball (enforced as a live ball) foul during the down.
Does it matter what we call the succeeding down?
If the team free kicks they lose the new (or old) series anyways.
By your logic ANY foul that results in a first down would invalidate the option, not just one that carries an automatic first down.