May be related to MC-7196. That issue is a closed bug so a new report is needed.

When caves generate in the Nether below the lava level, the caves are not aligned correctly at chunk boundaries. This is easiest to see with an external mapper that can map the world at arbitrary y levels.

The attached screenshot was produced with an external mapper. Seed is 1, mapping the region file at 0,0 to 511,511, at y=16. The chunk boundaries are shown with a lighter colour at x,z mod 0 and mod 15.

The issue can be reproduced as far back as 1.13 and possibly earlier.