Grimhaven boss arena bug

bug report reference number = 1413864673

after i quickly went inside the arena on the Grimhaven boss, i went out of it before it had time for the full spawn animation, when i came back to fight the boss i could not enter the arena due to an invisible wall, there is also no "arena" button that would let me travel in and out.

I have the exact same problem as a summoner. Some of my minions went inside before I did, and was then locked out of the arena.