It was more like this, I should clarify: on the top floor it was high spawn, on the second floor it was high spawn, but there's less enemies down there so it seemed like they were fading away. Then on the third floor there was just nothing. No loot, no enemies, but locked & trapped chests abound.
The character attempting the disarm didn't have rogue levels, but has disabled DC 35+ traps before. These traps were DC 40. I believe the DC 35 rule only applies for trap detection, at least according to the
NWN Wiki, but this wouldn't be the first secret the game keeps from players. (e: nevermind, found the answer
here, but that doesn't explain how they got into all those other locks)
I was told once that the entire dungeon's spawn will change based on what is killed inside, accounting for losses. Whatever the case may be, I don't think it was a conscious design choice to make "deep" areas of dungeons not respawn at all, despite the areas above them being fully respawned and elevated to max spawn. We killed everything we found, progressing slowly through the dungeon at a walking pace while talking the whole way, so that should have been
more than enough time for the areas below us to respawn. It was at least a full hour on the first floor that most people probably clear in 10-15 minutes, for reference on how leisurely our pace was. The only thing we got were locked, trapped chests, with no loot in them, and bookshelves that were empty.