Percentages etc are deceiving here. A 50% bonus is not the opposite of a 50% discount.

Your stake should grow or shrink by the same factor if you win or lose.

If you win, your stake is multiplied by 1.5.

If you lose, it should then be divided by 1.5 (multiplied by 0.66..).

But is in fact multiplied by only 0.6.