Even it sounds weird but the timer counts up if you play alone + it is mentioned as "Singleplayer" internally.
How does this work?
The menu UI disappears too slow making you able to press buttons you are not allowed anymore on game start normally.
You can play as any Monster too but you won't be able to finish the game.
If playing as prisoner, you can finish the game as normal or you fail.
(What renders no monster in the cutscene)
How to do that exactly? (Easiest way)
As Monster:
1.Host a game
2.Select any Monster
3.Have another player playing as a prisoner
4.Go both ready
5.Once the timer hits 0 the other player needs to spam the Leave Lobby Button
6.Now you can play alone as a Monster
As Prisoner:
1.Host a game
2.Select the Prisoner
3.Have another player playing as a Monster
4.Go both ready
5.Once the timer hits 0 the other player needs to spam the Leave Lobby Button
6.Now you can play alone as a Prisoner
The game can kick you randomly possibly because of some side checks.