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.