While playing singleplayer i emerged from a cave to find the rain texture had been replaced with textures from various files including the particles file.
Upon switching away from the game to a different tab and returning to the game, the texture had returned to normal.

The rain also rendered behind water during this time, this returned to normal with the texture

Possibly related: The game was struggling to render chunks close to the player for a few hours until the event took place (chunks would toggle in and out of visibility as they were updated by blocks breaking etc)

No modifications have been made to the game files, and nothing has happened to them that would lead me to believe the data has become corrupted.

I do not know what caused this, nor how to repeat it.