Zalmarath, the colossus - temporary unhitable bug - happened once.

During the battle (one of a few) after moving to the side position, the boss was unhittable - no damage at all - as seen on the screenshot (orb at point blank and no damage ticking up) lasted for that entire cicle and self-fixed when the boss moved back to the top area.. despite the repeated attempts / battles, the bug did not happened again.

https://steamcommunity.com/sharedfiles/filedetails/?id=3411556972