This is a long-standing bug that I’ve posted about since at least 11.0.5 which is usually caused by killing the trash too fast; specifically, the lieutenant lich after Vigilant Guardian who normally otherwise teleports behind each trash pack after taking a certain amount of damage. Sometimes he still refuses to teleport even when damaging him slowly with auto attacks, at which point I pull every remaining pack and try to kill them before killing him.
It doesn’t always work 100% of the time, but that’s my best workaround. When this occurs, the NPCs may appear to not move, but then after about 20-30 seconds they will teleport forward and begin running into position by the flight path.
However, there is still a very slim chance here that Bolvar will then bug out again and run all the way back to the hallway entrance, so I sit outside the hall on my mount waiting for them to teleport forward so I can talk to him immediately before there is a chance that it bugs out.