Using a portal after killing a boss can reset the boss.
After killing the boss for the level 75 island (taking the path to 80 island) and collecting loot, I used a portal prematurely before completing the quest by talking to the NPC. I was able to take the portal back, but the boss had reset and allowed me to kill him and loot him again.
Update:
Can be abused even further, tp’ing out of the area will trigger the removal of modifier’s similar to how they would be lost on death. If you’ve defeated the boss, it will respawn without the modifier. Yet to test, but I imagine if you tp in and out during the encounter it will reset it and move the modifiers.