This bug can also change the environment, meaning "Category: Rendering" no longer quite fits.
Encountered in latest dev version 20w20a.
Upon loading a saved game in the nether, all lava falls turned to stone and the world became noticeably darker. Screenshot ist attached.
Edit: wrong information deleted
My bad!
After restoring an old backup and revisiting the place, I see that you are correct.
I was certain those were lava falls before, and the rain had converted them to stone.
I have deleted the screenshot and will edit my other comment.
Sorry for the false alarm.