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:
Find a mob which can pick up and use armor (in this case the Wither Skeleton was summoned using /summon wither_skeleton ~ ~ ~ {CanPickUpLoot:1b})
Give it a soul speed pair of boots
After a few minutes of waiting you can kill the Wither Skeleton, revealing that durability has been used up.
Modded versions (even if "shouldn't affect it") are not supported.
Helmets lose durability on mobs when they burn in the sun, elytras lose durability if used by mobs, crossbows break when used by mobs. It's inconsistent, but those cases are by design, so I'd rather think that armor not consuming durability with "normal" usage is the issue instead.