When a Mimic is tied to a fencepost via lead, it can still be picked up. This leaves the leash knot on the post, but when re-placing the Mimic the rope is no longer connected, losing the lead item in the process.

Ideal fix would be to drop a lead (if attached); and optionally remove the knot as well (though this may be more involved as other tameable entities may also be leashed to the same fencepost).