If you attach a lead to a mob, and then leave the game without attaching the lead to a fence or dettaching it, when you join the game again, the mod and the lead will dissapear completely.
Note: this rarely happens. Most of the times, the lead is on the ground when reconnecting.
Is this still a concern in the current Minecraft version? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.
Note: this is a automatic message, so apologies if I miss something.