Boss spawned in a place he couldn’t get out of, causing the level to be unwinnable.
I recorded a clip (a bit longer than necessary because of an ambush, but I wanted to have it uncut to show there were no shenanigans). Uploading it to youtube now, because it was a bit too big to post here.
Heard the boss spawn before dropping into the basement, but after waiting 30~ seconds it never appeared. Finally moved on, but could hear it growling and thumping. It sounded most clear in the area where you come out of the basement, but neither bombs nor trying to attack through the wall would let me hit it.
Reproduction Rate (Choose One):
This was in the unmodded realm. Only mod that modifies in-game behaviour was the Better Bots mod.