When a player is flying unusually high (Ex. Y=9999999999999999999999999), the player is no longer visible to themselves (as far as I know) after changing the camera view by using the F5 key.
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[media]

The camera view is set so the player can see themselves in the screenshot. However, the player cannot be seen and the game does not throw an "illegal stance" error.

Duplicate of MC-45046

I know that now.