Gate doesn't open even when all mobs are defeated

During "Surviving The Ambush, The Nameless Kingdom" where Gate is blocked until all mobs are defeated.

-One of spawned mobs with spear & shied got stuck just behind lower end of the stairs next to the hole in the middle. I pushed the stuck mob into the hole. Its the position where the player character stands at the end of the attached video.

-Game doesn't recognize that the mob that fell into the hole is dead.

-Gate doesn't open even when all mobs are cleared.

-Only option to proceed is to quit & restart the mission.