I don’t know if this is a bug or not. But when you have the maximum amount of pixels you want to trade to and then invest more pixels than needed, then the ones exceeding the limit vanish.
Example:
Wanting to trade 1,500,000 white Pixels for 1,000,000 black Pixels. The 500,000 white Pixels that exceed that limit vanish. (If you have a 1:1 trade in this case).