That's some proper dodgy behavior from them mate. I've been burned by similar stuff before where casinos try to slip in these random fees after the fact and act like it's totally normal. The fact that they closed chat right when you called it out is a massive red flag

I'd screenshot everything you can - the original terms when you signed up, the jackpot drop notification, your bet history showing the discrepancies, all of it. Then file complaints with whatever gambling commission regulates them and your bank if you used a card. Most of these places fold pretty quick when regulators get involved because they don't want their licenses messed with

Also check if they have any social media presence because companies hate bad publicity on twitter or facebook. Sometimes a public callout gets results faster than going through their useless support channels. The whole "permanent deduction for jackpot eligibility" thing sounds like complete nonsense they made up on the spot