It's because of the scarred trait. I don't know the exact number for sure, but it seems to lower the base beauty of a slave by 30. So if you find a slave that would have been 19, minus 30 would make that slave -11. I'm saying it lowers the base beauty by 30 because -30 is the lowest I've seen.
All that aside, I don't know if the game is supposed to go below 0 and into the negatives, just my assumption on why.