I'll don't have a screenshot from yesterday, so I can't say with 100% certainty that this is the case, so I'll take your word for it....seems odd though that you'd have to do finish the objective 6x (once to open it + 5 more), when it could just have either said "defeat 6 bosses" or just popped up on it's own without having to trigger it.
I get it now...still think it's weird - just give me the objective and let me hit it or not....seems every other objective works that way.