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It could be understood as a (not very good) joke along the lines of this https://commons.wikimedia.org/wiki/File:FoundX.svg

Exactly. And it happened multiple times on the original tweet, where it also replied "fifth" when asked for the fifth word of the phrase.

Feels a bit too consistent of a mistake.

That may be what is happening. I rephrased the question and chatgpt got it correct.

Q: which word occurs third in this sentence?

A: The word "occurs" is the third word in the sentence "which word occurs third in this sentence?"