I actually did that same exact thing on my first trade...
I sold cBis at .24 then bought back at .17 and my cost basis was then .21 I had to call them to find out why and yup... they replied its one of those things you don't find out until you do it lmao - thanks fidelity for the good faith violation on my first trade!!!!
but now I'm not selling until I see some real green.