Sale of principal residence capital gain math seems to be broken. I set my ACB to be the same as the proceeds and it says I have $16,000 in capital gains. What?

I got suspicious when my manual calculation of capital gains from using CRA's form T2091(Ind) gave me a capital gain of $3500, then TurboTax said it was $16,000. So I ran the test where I set the ACB to match the proceeds and it didn't give me a zero capital gain as the answer. I don't understand what math Turbotax is doing. It is assigning me a big capital gain that doesn't exist. Am I missing something?