Unbreaking enchantment doesn't reduce damage from Soul Speed properly

The bug

It seems like the Unbreaking enchantment dosen't reduce the damage from walking on Soul Sand and Soul Soil using Soul Speed boots nearly as much as it should.

Video

https://youtu.be/QdKK62gkHBQ

The difference between no unbreaking and Unbreaking III is only 13 damage points, despite having traveled 1000 blocks.