Bug Report - Killed Boss Outside of the Arena

BugID - 2,914,602,998
Party: 3 player
Cause:

One player Leroy Jenkin's into the Boss area prior to the other two players arriving. It was the Bone Tyrant in the Hidden Grotto T14.

The other two members arrived to the fight, and were unable to join as the Boss had spawn. Sir Leroy died for his sins.

The other two members stayed outside of the area (as it wouldnt let them in) and in the spirit of frustration, my Merc Tactician threw down his Artillery Balisticas and they were able to spawn thru the barrier into the arena, and then over the course of ~10 minutes (with one or two trips to town to refill the mana potions of course) the boss died.

98% certain this is a bug, but kinda unsure WHICH PART is a bug.

If we are supposed to be able to kill the boss outside of the area, please fix so that my fireballs can pass thru it, because the area wall was stopping projectiles :(