When turning a Resource Pack on, randomly rotated textures – like the top of sandstone – are no longer randomly rotated. I double checked the resource pack and it has no files relating to sandstone in the models or blockstates folder. It's worth noting that other randomized textures like sand and the top of grass aren't affected.

This doesn't occur with every resource pack, but the ones it does occur with don't seem to have any similarities. It probably doesn't have to do with changing a parent blockmodel or blockstate; I deleted the models and blockstates folders from a resource pack that produced the bug and it still occured.