Tested setup (test environment):

• Tested with a pig on an oak fence in a superflat world on grass.
• Disable vanilla datapack or remove fences.json from the standard datapack (by making a copy, then just removing the single fences.json file. and disabling vanilla, enabling the modified vanilla pack only removing the one file.)

Expected:

• Lead works normally, attach to mob and fence and it will stay, even if we switch vanilla datapack off, as nothing within the vanilla datapack describes leashing of entities..

Actual:

• Lead breaks after few seconds, no matter how many times it is reattached, even though the datapack vanilla is switched off.

Reproduce:

• Tie a pig to a fence using a lead.
• Disable the vanilla datapack (/datapack disable vanilla)
• Observer as the lead snaps after a few seconds.

The reason for the bug:

• The bug occurs, because leads are internally reliant on the tag for fences.
• If the lead is connected to a non-fence block. The lead will auto-update and find itself to be not connected to a fence.

What needs to be fixed:

• Lead attachment should not be tied to fences.json and always stay attached when attached.

or

• The fences.json needs to be consistent as to disallow placement of leads on non-fence blocks to begin with, then also disallowing the staying of leads on non-fence blocks.
• Also, any new additions to fences.json (aka adding cobblestone to the tag #fences, should in theory allow the player to attach leads to cobblestone (although weird) and the lead should persist, as the block is tagged with #fences.

The latter is inconsistent with other tags etc. however.