The particles found in nether exclusive biomes are coming from points that lose floating point precision at high coordinates. This is rather tricky to spot in vanilla, but is definitely present (the Basalt Deltas biome in particular tends to show lines over distances).
This is best tested when greater than 16,777,216 blocks out on both axes, so being 20 million blocks out should suffice. This issue is without a doubt due to incorrectly casting to a 32-bit float from an integer or double, resulting in numerical precision being lost. This can be fixed by simply not casting to a float, which eliminates any chance of losing any precision.
Related tickets: MC-182789
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No that has nothing to do with it, since it also occurs at any y coordinate given extreme enough xz coordinates.
Using the following mod for 1.16:
https://www.curseforge.com/minecraft/mc-mods/farlands
I can confirm this does indeed happen, but is just difficult to notice within vanilla bounds. So my sightings within vanilla were indeed correct.
Can confirm, but possibly this is not caused by precission loss, but rather by MC-170823. I am also seeing these particles at low coordinates.