What I expected to happen was...:

By modifying the leash with /data modify on an entity without leash, no lead drops.

What actually happened was...:
When I modify the leash with the following command, a lead is always dropped (from mobs)

Steps to Reproduce:

As MC-179815 remove the leash with:

execute as @e[type=!player,distance=..40] run data modify entity @s Leash set value {}

This can be solved with MC-179817