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If I understand correctly, the default is no key change notification. So for the majority of users, MITM would go unnoticed here.

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I believe that you get a key change notification, but by default it doesn't require any sort of confirmation and will just continue to work with the new key.

GP is correct: no notification of key changes by default.

Even if you enable the key change notification, it is "non-blocking" in WhatsApp as outlined by the blog post: when you get the key change notification, the WhatsApp client will automatically without user intervention resend undelivered messages (unlike Signal, btw, from what I understand).

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