Stygimoloch Warrior instakills without hitting any traps or AoEs

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Date & Time: 5 June 2021
Frequency: Twice, once per session
Monster name: Stygimoloch Warrior
Class/Level: Black Mage, Level 80
Area and coordinates: Delubrum Reginae (Savage)
Steps:
1. Start the duel against Stygimoloch Warrior. Cast the ability Leylines after dispelling the boss's buff in the far north of the arena. The mechanic this bug happens on is Entrapment, all previous mechanics work fine.
2. Upon receiving knockback from the Entrapment cast, IMMEDIATELY use Between the Lines to cancel the knockback and return to the north of the arena, outside the trap zone.
3. The player will die instantly without having been hit by any AoEs or even taking any damage, which is contrary to how the encounter worked prior to 5.55. If this is intended, it was not listed in any of 5.55's patch notes or hotfixes.