By adding a lead to a mob (with item or with command) a leash appears.

What I expected to happen was...:

By removing the leash with data, The leash is removed

What actually happened was...:
But when I remove it with the following command the leash is always here (in data of the entity and visually)

Steps to Reproduce:

add the leash (or with item):

execute as @e[type=!player,distance=..10] at @s run data modify entity @s Leash.UUID set from entity @p[distance=..40] UUID

remove the leash:

execute as @e[type=!player,distance=..10] run data remove entity @s Leash

Update 1.16: the UUID system has changed in 1.16