Unlike all other types of armor which don't lose any durability once on a mob, soul speed is the sole exception to this where it's usage does lose durability. This makes it the only armor type which breaks when used by a mob, which is inconvenient and inconsistent.

Replicating the issue is simple:

1. Find a mob which can pick up and use armor (in this case the Wither Skeleton was summoned using /summon wither_skeleton ~ ~ ~ {CanPickUpLoot:1b})
2. Give it a soul speed pair of boots
3. After a few minutes of waiting you can kill the Wither Skeleton, revealing that durability has been used up.