This happened to us as well on Xbox One.
Multiplayer. 3 of us. Default medium stress level. All options default.
We found Burg.L within about 1-2 hours of our first play. We received the 3 quests. Nobody hit him.
As we were leaving there was a spider just sitting outside the very front door of the lab. Eventually it got into the first hallway of the lab. Killed us, we respawned outside and continued and finished our quests. Saved our game.
Next physical day (not game day), loaded up our saved game to play some more. Went back to lab, spider was gone but so was Burg.L. I have a quest to turn in one of the chips to Burg.L but he's not there in the lab at all anymore.
My daughter swears the spider must have killed him, but, we played another 5-6 hours yesterday building a nice fort, I'd hate to give up all that work and have to reset our game, especially if this can just happen again.