did you enter an amount for the extension and just not pay it? if so, Turbotax may have included that as an actual payment. if so it will show up on schedule 3 line 10. you'll need to go into your return to delete this "payment". if you did not fairly accurately indicate the balance due on the extension, the IRS can invalidate it. in this case, we have no way of knowing why your balance due dropped to zero. we can't see your return.
