This happens to me every year, but when I go to enter my 1099-B, turbo tax either double counts the income (already reported on my W-2) or assumes the entire thing is a gain. For example, I just uploaded a 1099-B for an immediate sale (upon vesting) of $24k of stock, and it adjusted my return down by about $13k. But these were shares sold SPECIFICALLY to pay taxes on the grant and had no loss or gain. Why does turbo tax think I need to pay $13k of taxes now?
For RSUs any amount on which you were already taxed elsewhere on the return gets added to your cost basis for the sale. If your vested price per share was $30 for example, and you sold 10 shares to cover tax, then the cost basis of that sale is increased by $30 X 10 because that's the amount of tax you already paid on the shares you sold.