I have this same problem with a trade in an imported broker's 1099 (interview method not used). (I did not tell TurboTax that I didn't get a 1099.) In TurboTax the trade correctly shows Box B applies, but the resulting Form 8949 categorizes it as Box C.
A possible clue to this apparent TurboTax error: This trade is unusual in that it involves a publicly traded partnership for which I received a K-1 form. I can try deleting and reentering the trade, but if this is a TurboTax error, it needs to be fixed.
