The bug
When you use a single lead to leash an entity the item is removed from your inventory. If you have multiple lead items selected this does not happen.
This was not the case in 1.10.2
Related issues
relates to
Comments

It seems that this doesn't happen if you hold more than one lead.

Is this causing MC-105979 ?

Confirmed fixed for 16w32b
Confirmed. Only occurs when you have one lead in your hand.