The wool purchased by the shepherd was changed in color by mistake

After my friend and I dug underground for a long time, I found that the wool bought by our two shepherds who had traded turned black.  ( They haven't buy black wool at first, one is white and the other is brown)There're less than 10 white and brown wool in the chest .We were underground for about an hour.
Expected Results: The shepherds still buy white and brown wool
Observed Results: The shepherds buy black wool