If lines 13 and 15c of Form 8606 have asterisks next to them TurboTax is doing the calculation on Worksheet 1-1 from IRS Pub 590-B and is properly leaving lines 6 through 12 of Form 8606 blank.
If TurboTax was simply failing to account for the amount from line 18 of the IRA Information Worksheet, TurboTax would be calculating a taxable amount that is too low, not too high. It's quite likely that the taxable amount that TurboTax is calculating is correct.
