Details:
This appears to happen when two players are involved.
What I expected:
Once the arena door closes, the fight should fully lock the encounter so players outside cannot keep boss aggro or damage Richards through the gate.
What actually happens:
Richards can still stay aggroed on the player outside the arena. Because of that, the outside player can attack and damage Richards through the gate with a long reach two handed weapon while the player inside keeps Richards near the door and avoids attacks.
Possible reproduction steps:
Start the Richards fight with two players.
Let Player 1 enter the arena and move to the far side away from Richards.
Keep Player 2 outside the arena.
After the door closes, Richards can still stay aggroed on Player 2 outside.
Player 2 can hit Richards through the gate using a long reach two handed weapon.
Player 1 stays inside and keeps Richards close to the gate while dodging.
Even if Player 2 dies outside, they can return and continue until Richards is killed.
Impact:
This allows players to kill the boss in a way that does not seem intended and bypasses the normal encounter.