I remember seeing similar cases on other casinos where they'd have a limit on how much you can get out of their bonus.
Even if the bonus they're giving out doesn't seem to be fair, there's nothing we could do against the terms and conditions of their bonuses since it's their casino they can also update it at any time. If you're hoping for a better bonus then it's best to not take the current bonuses they offer and hope the other players do the same so they'd get the message and make some improvements.
Most casinos do really set limit on how much you can get on their bonus because after all, it is just a type of promotion to gauge the players into playing and depositing. They still have to profit out of it since they are also a business trying to thrive and generate income as well. It is not really a secret that these casinos gradually lower the RTP once the players gets to play and win consecutively. I believe this is also included in their terms of service so you must really read and digest all of their policies before proceeding to avoid trouble. Otherwise, you will really experience inconvenience because you don't know some of their rules.
If the casino didn't put that on their TOS, then you can raise a concern regarding the sudden change/s made into your account. However, it is also known that some casinos change TOS without further notice, so it will be your obligation to check if they have made some changes on their terms once in a while. Some send alert messages while some don't so you have to rely on your own if that is the scenario.