There’s a thread on the CS board discussing it with a possible solution that I haven’t had chance to try yet, but is supposed to work.
Apparently Bolvar is there, he’s just got a stealth buff! lol
EDIT: Have now tried it myself and this works. Basically, go and get the Spyglass item, it’s in a chest in Maldraxxus, it lets you see him. Absolutley barmy.