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.
Related tickets: MC-182789
Linked issues
relates to 15
MC-76810
Casting issue: Bone meal particles break at high coordinates (floating point precision)
Resolved
MC-161969
Casting issue: Adding ender eyes to end portal frames at high distances causes particles to lose precision
Resolved
MC-164352
Casting issue: Snow animation becomes stretched when being very high above the ground
Open
MC-167046
Casting issue: Lava ember particles and lava popping sounds lose precision on creation at high coordinates
Resolved
MC-167047
Casting issue: TNT blocks ignited by other explosions lose precision at high coordinates
Resolved
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Due to MC-164158, lava didn't produce any smoke particles when being hit by rain. Maybe that's why you didn't notice it before.