On our survival 1.9 server, running Spigot 1.9 from Jenkins build 677, we're finding that leads are not dropped after use on horses or donkeys. Tested both with stretching the lead too far, and right-clicking to detach it - in both cases the lead just disappears, rather than dropping at the animal's position as it ought to.

I tried in vanilla MC 1.9 in singleplayer, and the leads drop as expected.

I've also reproduced the issue just now on our test server, onto which I just built and deployed a build of Spigot produced just now via BuildTools, version git-Spigot-b1c1b55-7d330bc

If I had to take a guess, I'd say maybe it was introduced in commit bf43ca62a65 - but that's sheer conjecture at this point.