Lifted to a height of approximately 100 units, there is an artifact of the skybox, a black bar appears, which increases with the rise up.
Sorry for my bad english.
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Normally, that area should be light blue (fog):
[media]What happens if you set your Render Distance down?

At low render distance - disappeared altogether the sky, at the maximum - texture chaotic start to run

Dupe of MC-62175