Why would having a stock sale with a big loss result in my "Obamacare premium" not getting refunded, even though I have a "taxable income" of $0?

If I fiddle with the "cost or other basis" making it look like I made *more* money on the sale, tt shows the premiums getting refunded.  But if I show the full loss, it sets the refund to $0.  Does that sound right?