Damage to the durability of boots caused by running on Soul Sand is not prevented by the Unbreaking enchantment at the expected rate of 75% for an Unbreaking III enchantment.

Reproduced by running along a 350-block-long strip of Soul Sand with two different pairs of boots. One with Unbreaking III and Soul Speed III and one with just Soul Speed III. Then checking the durability of each. Tested three times and each trial resulted in both pairs of boots being within three durability points of each other.

Expected Behavior is that Unbreaking III would prevent damage caused to boots by walking on Soul Sand/Soil by about 75%. But they take about the same amount of damage, rather than the one without Unbreaking taking four times as much damage as the other.

This results in Netherite Boots with Soul Speed only lasting about 45 minutes.