Priorities are global across the entire Jira instance. Please do vote for the issue @verraf linked. Your vote matters to having things like this fixed eventually in Jira!
Jira admins should work together to avoid an excessive quantity of values for global things like Priority, Status, Issue Types, Custom Fields, etc.
I've even seen Jira instances with both a "Canceled" and "Cancelled" status. Like, really??
I'd have a conversation with all the Jira admins to 1) keep the problem from getting worse over time, and 2) see what steps can be taken to reduce the problem now.
Other than that, the only options I can think of are after-the-fact. You could use automation, for example, to "undo" a change to the Priority field for a certain project. This is likely to be annoying to users, and admittedly is a crappy solution. But it would enforce your policy.
Be warned that if you are new to automation, it will take some time to figure out how to access the changelog in Smart Values to do what you want. But trust me, this is feasible in automation -- just not trivial.
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