I wanted to come to my world of version 1.8 with a size of 3.78 GB, which I successfully converted to version 1.12.2, through the new version 1.13. When I did everything and went out into the world, I found a bug of chunks. Also when you hover over the world icon it says"dont forget to backup this world before you load it in this snapshot". Please help to solve this problem.
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Looks like MC-133855.

Yes, this is a similar situation, but the person who wrote it talked about almost complete replacement of the chunks, and I have new chunks are generated by long cracks sometimes connecting.