I tried doing this escort quest in Stormwind, which is a part of the Onyxia attunement questline. First time it failed because Bolvar wasn’t in the keep once i got Windsor up to face Onyxia. Second time it failed, because Bolvar just disappeared after fighting Onyxias Elite Guard for 5 seconds. After his disapperance the Elite Dragons just went berserk and attacked whomever was closest to them.
I’m going to try again in the morning, but with 20 years of bug reports on this very quest, I don’t think it’s too much to ask of Blizzard to actually fix this quest once and for all.