How to reproduce

Cheat on

1. generate one or more pigs, tied the pigs to a fence
2. (Use carrots to lure them in a direction)
3. use /tp command to teleport the pigs in the opposite direction that they are walking to, while teleporting far enough to break all the leads that tied on the fence

Cheat off

Allay can break the leads on their own due to MCPE-159616 / MCPE-158955.

So when you tied lots Allays to a fence, trigger that Allay escape bug, the last few Allays break the leads at the same time may cause this bug.
this is also mentioned in the comment of  MCPE-158955 Allay Unleadable

What happens

leads don't drop. pigs still have a leash attached on them that other side not attached.

those pigs cannot be leashed again (and the leashs visually gone after the chunk where the pigs are reload).

the leash knot on the fence disappear. (in java edition, the leash knot keeps, all leads drop, pigs can be leashed again, but there's a open issue for the not disappearing leash knot MC-96347)