I've sent 2 big bugs like this, one Funimation and one for a dating app.
Funimation you could access anyones PII and shop orders, they ignored me until I sent a linkedin message to their CTO with his PII (CC number) in it.
The "dating" app well they were literally spewing private data (admin/mod notes, reports, private images, bcrytped password, ASIN, IP, etc) via a websocket on certain actions. I figured out those actions that triggered it, emailed them and within 12 hours they had fixed it and made a bug bounty program to pay me out of as a thank you.
Importantly, I also didn't use anyone else's data/account, I simply made another account that I attacked to prove. Yes it cost me a monthly sub ~$10 to do so. But they also refunded that.
Been there. Nagged the city of Seattle for nearly two years about fixing their insecure digital wallets, and in return they just acted weird to me and never really fixed the problem. Wouldn't tell me anything not even the vendor so I could communicate to them that this issue could exist elsewhere. The goal of these tactics is to delay long enough that you give up on publishing. So publish. Just be ethical and stay within the bounds of the law on what you access and release.
I did a quick test and seems like the full admin access that I used to get is slightly fixed/changed. I'm wondering if there was an issue and I have enough data to show there were full compromised of all users data, but it is changed now (might still be vulnerable but let's say it's not). should I still release something? they should have notified their users of such an issue right?