No, it's definitely not a visual bug. I'm not exactly an expert in this matter. they have a problem with buying a bowl game, you can pay less than what the system asks to do. and this will clearly affect the RTP of the slot itself with a long distance.
This seems like a critical vulnerability! Any decent game provider (or any developer in general) is supposed to give your report the highest priority and will try to fix the bug or patch the vulnerability as soon as they know about its existence since it may result in severe financial loss for their customers (casinos).
I suppose those game providers get tons of emails every single day (many of them are spam) so make sure to send them a detailed report with a PoC to explain how the vulnerability you found can be exploited and its impact.
Instead of ignoring your messages, they should give you a reward.
I guess this need to make an action by the gambling platform immediately because this is a crucial related to their transactions it might affect the sales and if the people see this kind of bug they will take an abuse and make an easy money. Do you have a response to the team already?.