No, he isn’t the first. Chester Arthur’s father was not a citizen at time of birth.
And we can thank Leo Donofrio for finding that...in 2009...
Arthur hid the fact and hid it well due to the implications.
And Arthur’s actions to hide the truth, appoint a SCOTUS judge to override the law, and destroying his White House files upon leaving office tells us old Chester knew he was not Constitutionally qualified and he wanted to ignore the Constitution, just like the current crop of democrats and feckless pubbies.