My wife and I had this glitch today (Jan. 2021). After reading this post, we restarted the mission and just slowed down. The first time we jumped right into combat and annihilated everything before it had even finished spawning. I think the reason everyone is having issues on the first 4 pilons is because they go to fast. After the 4th, veterans start spawning and they take longer to kill. Just make sure you slow down, let the nps's engage the mobs and then kill them. This worked for us on our 2nd attempt.
