How To Reproduce:

Create a world in 21w39a (it is also a problem in earlier versions I guess since y-biome was introduced like a year ago) with seed 6406292739340372647.

/execute in minecraft:overworld run tp @s -566.93 69.00 -482.13 -65.42 15.21

Look under your feet, there're raining particles on the ground but no droplets falling from the sky.

Fire can be extinguished by the rain so it is a real rain but has a visual bug.

Under the tree that you stand on, the grass colour is olive-green which belongs to savanna biome, but on the leaves the biome changes to forest. I think this difference casues the bug.

In the screenshot we can see a contrast between the visual rain in savanna-forest area and the pure forest.