The bug

When lava displaces a plant or another displaceable block, it renders smoke particles instead of spawning an item (like water does). These smoke particles seem to render too high. The bug is especially visible in the nether at the flatter lava blocks. 

How to reproduce

1. Make or find a fully spreading lava flow in the nether (for best reference, but overworld works too)
2. Place nether sprouts (which are small) in the lava blocks at the edge of the flow
3. Let the lava consume the nether sprouts and spawn smoke particles
   →  Smoke particles render too high above the displaced block
   (like they were the full block tall - the nether sprouts may have wished they were that tall )

In the screenshots, the lava nor the nether sprouts are high enough to make smoke at that location.

Before:

After: