cannot leave map boss arena

i just did RustBowl on 14 waystone,
it had rituals, breach, delerium And a boss
when i reached the boss i got in, to kill it, which i did
i opened a portal to hideout to clear bag space
when i came back, the arena portal would NOT LET ME OUT
so basically the map failed when it was almost 25% complete because the Arena would not let me out
it this a new mechanic to leave map boss to last or something ???
really frustrated here ....