Same problem here, On PlayerQuitEvent no teleport works.
Version:

git-Spigot-798f32d-0cd0397 (MC: 1.9.4) (Implementing API version 1.9.4-R0.1-SNAPSHOT)

For anyone reviewing this: I assume that the issue is related to a small change in PlayerConnection.isDisconnected():
'!this.networkManager.channel.config().isAutoRead()' was changed to '!this.networkManager.isConnected()'

And networkManager.isConnected() returns false already when bukkit handles the player quit after the player 'disconnected'. networkManager.channel.config().isAutoRead() still returned true in this case.

Possible solutions might be:

• Reverting this change: So any other server code which might rely on the old behavior would get 'fixed' as well.
• Remove the 'entity.playerConnection.isDisconnected()' check inside CraftPlayer.teleport(..). It might not even make much sense to check for that there anyways, because it already checks if the player entity is still alive, and the player entity always gets killed by the server after the server has handled the player quit (so this check might be redundant anyways).
• Add some sort of 'this.player.left' flag to the isDisconnected method, similar to how there already is a '!this.player.joining', which gets set after the player quit has been handled.

Bugfix would be very important for me!