I'm surprised a bug like this could exist if it does. I've played all the way back in NM4 and I definitely remember seeing those Altar Assassins. I even remember vividly back when I got trapped by both the DAs and bigger Altar Assassins in Chaos 5 during launch. I wonder how did you manage to beat that C5 incursion since clearing the wave depended on the Altar Assassins being killed after they become vulnerable as the wave are infinite.
Though it's possible you might indeed just be in the standard Expedition map. However, if those three times you've played were before Protean Shift in 2018, that's kinda impossible as Expeditions did not exist and Trials' Chaos 5 did not have Crumbled Bulwark in it's map pool back then.
Just my observation there.