Happened to me twice yesterday, have been advised to make a report here. Both times were on the 3rd or so boss in a BC raid while soloing them on a Level 80 Havoc Demon Hunter. Legacy loot rules were in effect, as usual when soloing old content as a high level character. All other bosses in both cases acted and dropped loot more or less as normal.
Both incidents were identical in terms of the end result: I kill the boss, get the same message you would normally get from joining a raid lockout in progress, and the boss was dead on the floor unable to be looted.
Incident 1:
Where: Karazhan
Who: Moroes
How’d I kill him: Double jumped and used Fel Rush from the bottom of the short stairs he’s at the top of, killing him in one hit.
Incident 2
Where: Serpentshrine Cavern
Who: Leotheras the Blind
How’d I kill him: Just walked up to the 3 mobs imprisioning him and activated Immolation Aura. This killed them instantly as well as him the instant he became active.