Floor Rearrangement

"Floor Rearrangement" is a feature of the personalization AI that rearranges floors from Princess Peach's Castle to a whole new order, as well as having brand new ways of accessing the upper (and lower) floors.

How it happens
The most common way people experience this is by wanting (and trying) to go to upper or lower floors without the appropriate amount of stars or key.

What's worth noting is that a lot of people playing this game would actually have the mindset above, so it's pretty strange that most people haven't experienced this anomaly at all.

Common Experiences
Most people who experience this usually start at the 3rd floor. The 3rd floor's appearance (in this case, the 1st floor) doesn't have the endless stairs. It also doesn't have secret stars such as "Wing Mario over the rainbow". The same people also have the 2nd floor as there basement. The 2nd floor (the basement in this case) has no stairs that lead to the 3rd floor (since it moved to a new place). The same people also have the main floor as the 2nd floor. The main floor (the 2nd floor in this case) has no stair entrance to the floor. However all secret stars are located here, for some reason. The basement is the 3rd floor and has the infinite stairs, and removes the pillars room.

More facts and info
If a floor has a very important room, such as a bowser course room, the AI will carve and create a room for that course. The AI will also do things such as make new rooms so the player can access the above and below floors. The AI will also shift around secret stars to make things less complicated. One thing though, after doing this process of floor rearrangement, it cannot make any more personalization (make new levels, rearrange levels, edit textures, spawn beta stuff, etc.)

Theorizing
Nintendo probably made this an AI mechanic due to them feeling that players will want to access other floors early.

They also could've made the AI do this because of testing, but they accidentally left it in the game

One thing that proves that they accidentally left this AI feature in the game is that if you do the thing that triggers the AI to do this phenomenon on a already personalized copy, the next time you load Mario 64, an anti-piracy screen comes up. Which, probably means the complete code was not finished and bugs would happen, alot.