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.
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bdm68
Attached is another image from the same world and location that shows the location of all lava below y=20. It shows that caves are replaced by a series of lava-filled caverns. It is not known if this is intentional. I assume it is.
The lava cave map also shows discontinuities at chunk boundaries. These discontinuities only appear to affect the larger circular caves.
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Still happens in 20w27a
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Can this be replicated in 1.16 or the 1.17 snapshots?
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Can be confirmed in 1.17.1.
j_p_smith
As of 21w38a, the circular caves now generate as ordinary caves, and are no longer cut off at chunk borders. If I had to guess I'd say the circular shapes were never intended to begin with, but either way this issue hasn't been reproducible for a long time.