It’s not something that follows grammatical rules. Languages all borrow words; if you watch the recent Ukrainian press releases, you’ll notice that there’s the occasional word that is exactly the same in English. They have an entirely different alphabet. Language isn’t law. It’s a living being.
Edit: I left out the specific point that most languages have unique cases for many words, borrowed or not. It’s just the way it is agreed to be correct by some organizations.
Edit edit: also, you literally described a rule with exception. the exact thing you disputed.
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