Where does the flat world start to corrupt?

Solution 1:

It appears your game is glitching, as flat worlds should be unable to be glitched drastically like that, while normal infinite worlds can. Additionally, Bedrock’s Graphics Engine begins to fail at certain distances, which is the main cause of the problem. According to the Minecraft Wiki:

Far Lands do not generate on flat worlds, due to the lack of a noise generator.

Normal worlds get corrupted because they are generated with something called Perlin Noise, which is often used to give a randomized, rough texture to what it is used in. But, after around roughly 12,550,000 blocks, the procedural generation becomes corrupted, as the number becomes too large for the game to handle.