If you teleport to for example x: 20000 y:70 z: 20000, the map generator doesn't generate anything, you fall down and die and loose the items, but that is not always, sometimes that happens
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I've just experienced that @Ali Yes, I know, but that is not good, if you loose your items after teleport
that is because the chunks need to generate first before you can teleport there, better just teleport into already generated chunks!