Well, I've definitely figured out the enigma that is Onana.
He conned United into signing him.
Have you ever noticed that his hairstyle is cut to stand about 3 inches off the top of his pip?
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It's been the same style since he signed for them, and it has intentionally been cut to look that way.
And no, it's not because he's trying to make some sort of profound fashion statement, or because he thinks it looks stylish, or even because he's trying to start a new hairstyle trend...
No, it's because that's the billed height he sold United on when they were looking at him as a potential signing.
It's clearly been an oversight on United's behalf, and he's had to live the lie ever since his signing, because nobody has called him on it yet.