When it rains on lava, the produced smoke ends up losing precision when far from spawn.
I remember explicitly testing for this late in 1.15's development and not finding it, so this bug has likely resulted from a very recent change.
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.