A hotfix for this has been pushed to beta (and will be on live as well).
If you having issues, please retest now. Dread-Rider Malwick should now respawn during the quest "Assault and Battery" about a minute after he despawns or dies.
so we tested this again now. if you aggro the Dread-Rider Malwick , and dont kill him, he will despawn but wont reset and the bat will stay flying bugged out without the rider spwaning back