Chunks moved to different locations when chunks were saved [1.15]

**Description

I upgraded my world from 1.14.4 to 1.15 and i generated new chunks in 1.15. I loaded and unloaded the chunks numerous time via nether portal. But one time a lot of the chunks in the region got moved to a different location. 

Behavior

Most chunks have been moved to another location, but i haven't been able to find all the pieces of my builds so some chunks may have been overwritten. Also the progress i had when going into the nether wasn't saved.

Steps to reproduce

I don't know the steps to reproduce this bug consistently, because i had already loaded and unloaded the chunks many times and i didn't do anything differently that time.

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