When you are about 2048 blocks away or more from your spawn point. You're able to fall through the world by being Exactly in the middle of the hitbox of the block, due to blocks not having the correct hitbox. As told in AntVenoms latest video https://www.youtube.com/watch?v=q3BvjYdqM0g
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However, this issue is a Duplicate of MCPE-39299.
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