Interesting. Are the ingredients each white or nearly colorless? If not, then can you use a better grade or purify? If they change colors when mixed, maybe they are oxidizing. One of the common ways to prevent fats and waxes from oxidizing is to use tocopherol, also known as Vitamin E. I said common -- it didn't say it worked well, in my experience it doesn't, but you can try it.
Also, when the appropriate amount is used, does it discolor the board? Or is it just bad for light colored boards? Or only dark in the tin?