unfortunately since this currently works by calling vanilla leash code on more or less whatever, the flaws of said handling are widely exposed.
the good news is they appear to be benign.
the bad news is that rendering of the leash breaks for things that are taller than two blocks, things that are shorter than one block. or things that update their movement in a way that bypasses the distance restriction as ghasts and bats do (or that have abnormal movement altogether like who knows what mod mobs).
Such mobs have varying combinations of of three flaws: ignoring being dragged by the leash, being unattachable to posts, or ignoring the leahs range limit (and often flying majestically toward the horizon until they despawn).

Potential fixes mostly involve writing custom versions of the functionality instead of hijacking vanilla's implementation, such as more robust position restrictions, estimating model centres to attach to so the line isn't just floating wherever, and possibly temporary ai alteration for support of particularly uncooperative edge cases.