now it's 2021 and this bug still applies.
After hitting the boss till its HP drops to exactly 25%, it's where the boss is stuck.
it just stays at the spot, becomes invincible ("Determined") with "Regret of Chaos" condition.
Edit: then I quitted the instance by running out of bound, then re-entered the instance (resuming from inside the city); but this time, the boss became un-targetable! I could not damage the boss, but the boss still performed attacks. This is so messed up! Please fix!!!