Recently someone did a juggernaut run where it was put under 2 teams and not FFA, you can even see it yourself when spawning into the juggernaut mode.

juggernaut works differently when u vote FFA. i recently had a safehouse juggernaut game where my lobby voted FFA and the teammates could damage each other.

That's a bug then.

bug looks funny me recently i killed juggernaut at same time he kills me i where stuck in spectator on him while i was a juggernaut spectator

Oh so thats what happened in one of my servers a while back, that was really confusing.

Normal juggernaut runs go into 2 teams and bugged runs that allow you to kill your teammates go into FFA. I might change that and remove the broken runs later