I came across this by accident and saw that there wasn't a duplicate bug (as far as I could tell) so I decided to report it. Essentially, attaching a mob with a lead to a fence and then getting the mob into a boat breaks the lead, but leaves the texture on the fence. So far I only tried it with cows, chickens, and pigs, but I assume it applies to all lead-able mobs. This has also been tested repeatedly and happens every time.