Just in free roam. Once when I was retrieving a phone recording in the club at the pier. Walked in, empty building, then on my way out I got taken by surprise when I turned around and the building was suddenly full of mobs. And again when I was trying to disable a jamming device in what looked like a mall, also in the new LZ area. Enemies kept spawning constantly and also right behind me when I walked a few steps forward or turned away.
Just booked it to the jamming device to destroy it since I was literally running out of ammo after about 10 minutes of non-stop assault, died right after because I was totally overwhelmed, returned to collect drops, and tried to stop the hacking attempt. Then, I was suddenly encircled even though I checked + cleared the area and was covering the only point of entry. I kept checking my radar to be sure I wasn't being flanked and I was fine until suddenly there was a dude unloading on me. It was on a rooftop and I couldn't find a way they could've gotten up the side of the building while I was checking the one entry point to the roof.
I distinctly recall something very similar happening to me several times when I played it around 1.8's release, and I got really frustrated then too.