I observed an issue on Old Haunts where Specials and Hordes stopped spawning. I had a hunch what caused the issue, and reproduced it. This was observed prior to the latest QoL patch, but I can confirm later if it is still occurring.
Difficulty was Cataclysm with Twitch Mode enabled.
To cause the bug, I simply flipped the Skaven portal regulator switch twice. The switch I’m referring to is the one for the Okri’s challenge that summons a set of scripted Skaven enemies. This is in the area of the map based on Summoner’s peak, prior to crossing the double bridge near the unmarked grave. For the challenge, you’re supposed to flip the switch three times, but flipping it only twice and leaving the area seems to block all spawns that aren’t ambient enemies, boss/patrol triggers, and then the finale event.