I'm not going to comment on the bug itself as no PoC has been provided (I don't think it's ethical to post it publicly without the casino's consent, anyways).
However, I wanted to salute you for your honesty and reporting the bug instead of exploiting it and milking the casino's wallets dry!
I also wanted to share my opinion on your second question:
How does someone go about reporting a bug to a casino a still hold casino accountable to pay a bounty?
You can't hold them accountable especially if they don't have a bug bounty program. Even those who have it but don't want to pay you can claim it's
a duplicate and reject your report.
that is true, check the section of the terms regarding bug-related protocols for the site. because if they have, they have certain requirements to comply with. because not all sites have their rules on this bounty. however, for ethical purposes, one can always contact their support and ask for assistance about such concern. it is your conscience that will direct you if you will tell them about your discovered bug.