MC-236687 was marked as "Works as Intended," meaning 3D biomes overlapping and such are indeed supposed to happen. But I can't be sure if that means foliage generating in biomes one would think they aren't supposed to is also intended.
The most common culprits of this are dead bushes and ferns. Usually a Badlands or Jungle will generate far above or below a biome, which is intended, but put its foliage on it even if the actual biome this foliage is in is something like a plains.
The actual biome the player walks on and sees generates the appropriate foliage for whatever biome it is, not that of another's.