played classic blackjack with bonus funds and cashier says that game was excluded

Ran into something weird with a bonus I got from a casino. Was grinding classic blackjack and thought I was ticking all the boxes on the T&C, but the cashier hit me with "game excluded, winnings void" when I tried to cash out. Never saw any explicit mention that blackjack was excluded in the promo details - if anything, their rules felt super generic and vague. Went through their FAQ again and now I'm not even sure what counts as "classic" blackjack in their system.

Is this just standard practice for sites to pull this kind of thing with bonuses? Anyone had a similar situation where they actually got their winnings back, or is it usually a lost cause once the cashier drops the hammer?