I don't remember how it was when I created my Punkz account (the casino was called CasinoPunkz) at the time, but they don't automatically assign welcome bonuses to deposits. There is usually an option to tick/untick or a button to click on to claim a bonus. You probably did that without thinking or reading what it does.
When you claimed the welcome bonus, Punkz added bonus money to your account. You then started wagering, which is required before you can withdraw. During this process, you won and lost bets. Your balance grew to 950 USDT. But this is still not your money because you didn't meet the wagering requirements. When you cancelled the welcome bonus, all the bonus money and the winnings that originate from bonus funds get removed from your account. That's why your account balance dropped.
You expected that your balance would go down to the original 500 real-money deposit. But it doesn't work like that. When you play with an active bonus, the casino first uses your real-money deposit for the bets you make. They can also mix bonus money and real money. This all happens in the background and you don't see that. For example, you make a 50 USDT bet. In the background, the casino takes 25 USDT from your deposit and 25 USDT from the bonus allocation. If you lose that bet, you also lose 25 USDT of your real money. You don't get that back after cancelling a welcome bonus.
The 200 USDT that seem to be "missing" from your account is simply 200 USDT that you lost while betting and trying to meet the wagering requirements.