The bug

Experience orbs don't collide with flowing lava correctly. They act as if the lava is a full block.

To reproduce

1. Place down lava using a lava bucket.
2. Set a repeating command block to summon experience orbs above flowing lava.

Observed results

The experience orbs float around in the air as if the flowing lava was a full block.

Image

Code analysis

Code analysis can be found in this comment.