Also, I noticed if you sprint past all the enemies on the first floor and don't shoot them. There won't be any enemies on the second floor.
I just tested it, I don't know if they corrected it but I dodged the enemies of the first floor and went to the bottom of the 2nd floor, the enemies of 2nd floor did not appear at the beginning but when I looked back, the enemies of first floor went up the elevator. Once I killed those of the first, those of the second came to life. So it works fine.