So having encountered the bug a bunch of times, and even found that it seems possible to trigger it 100% of the time i´d just like to mention that depending on where you stand in the finale area the bosses wont manage to enter the fight.
You can even lock out enemies and specials too, they all seem to struggle to enter when the team is around the temple door or on the balcony like walkways. Although only bosses get hit really notably bad.
I suggest giving the bosses a fixed spawn point that wont bug out in that place like knocking down a wall or suddenly jumping down from some of the upper balconies. Because this whole variable spawn according to team position isnt working out at all.
Only reason i didnt make this report sooner is because i pretty much never get the map in my QP selection relative to the likes of Enchanters lair or blood in the darkness which appear all the time.