Leads won't keep hold of mobs when chunks are reloaded

In my single player world, I have tied mobs to fences using leads but then once I leave where the specific chunk till when it unloads and then go back to where the chunk is reloaded up, the lead is present on the fence but is no longer tied to the mob

I expect that the leads would keep the mobs tied to the fence and not allow them to be free once tied.

I left the area where the chunk unloads out from my render and then return to where to chunk reloads finding that the mobs are no longer tied to the fence though the leads are still present

Steps to Reproduce:
1. Attach any mob to a lead and tie it to a fence
2. Leave the specific chunk till it unloads and can no longer be seen from your render
3. Return to where you have tied to mob and check if it is still tied. The lead will still present on the fence though the mob will no longer be tied to the lead (This doesn't happen every time but it does happen pretty often for me)

Thank you.