If the issue you found is “expected behaviour”, then there’s no harm in sharing it. Do it publicly while mentioning your timeline and their response. Let everyone else decide if it’s truly an issue or not. If they end up changing it, it becomes proof you found a legitimate problem. That doesn’t guarantee they’ll pay up, but they’ll get even more bad press if they don’t.
Apple has already indicated they don’t intend to pay you. By keeping the problem a secret you have no recourse and will continue not being paid. Unless you know someone at Apple which could make it happen, anything other than sharing the bug is a waste of your time and presumably harmful for users who won’t know of the problem and thus can’t protect against it.
¹ https://www.macrumors.com/2021/09/09/security-researchers-ap...