In my opinion it's a substantial bug but again I'm not qualified or educated enough on it. But if I can exploit it then others would have fun. Very reputable casino with a large reach. I've contacted support and waiting. No bounty page on their site but for sure reputable so I hope they would compensate given the bug.
looking forward to what they have to say when you show them what bug you found, I am also curious about which casino you found this bug on and what issue/exploit the bug is causing(would you mind sharing what casino it is? you don't have to if you don't want to share the name of the casino).