It's hard to tell, because there are other ways to obtain that kind of sitting, but that's the only way that softlocks it.
Basically you can obtain that same type of sitting by using an effect in a chair like frog, exiting the chair and using frog so you can walk around while you are supposed to be sitting. When you do this you have around 12.5 second before the game will force you to sit down. When the games timer ends and you are forced to sit down you will be sitting with the spite of Madotsuki with her head down. Which has a lot of interesting properties.
The reason for this I am not really sure of, but because of the fact that the sprites existence can already be explained by the Madotsuki in the closet(which means it wasn't custom made) that means that it is more likely to just be something along the lines of the game not knowing what sitting state you should actually be in. There is evidence of this because if you use certain glitches to be able to have the sitting sprite while moving it will cycle through the 4 different sitting sprites used for the animation if you have the normal sitting state. However if you have the head down sitting state it will not cycle through any different sprites while you move.
Plus why would someone purposely put an event in to softlock their game? It would ruin any save progress, and just doesn't seem like a practical design decision.
If I find anything more about it I'll post it here.