Update:
Moving the landscape that doesn’t work to a level where it does work allows me to paint foliage as expected. Also, creating a new landscape in the level that doesn’t work still gives the same result where the foliage (other than the rocks) doesn’t show up. This leads me to believe that it’s a level issue specifically, but I honestly can’t figure out what happened. Is this a setting I accidentally disabled, or Is that entire level just buggy now?