Any winnings received from a bonus bet will be minus the initial stake amount!
When you place a bet using a bonus bet the winnings are calculated differently to a cash bet. A bet placed with a Bonus Bet will pay out the winnings minus the bonus bet stake.
An example is as follows: If you placed a $10 Bonus Bet at odds of $10 your winnings would be $90.
This is calculated by multiplying the stake by the odds ($10x $10 = $100) and then subtracting the stake of the bet ($100 - $10 = $90).