Recently when I imported a script intended to turn off player collisions, some stuff was deprecated so I removed it.
From that point on, Roblox would load R15 instead of R6 despite me clearly selecting R6. Even stranger, Player Choice would load R6 just fine.
Even after reinstalling Studio, the issue persists across ALL my places, regardless of if they are new or old.
I also managed to find the script that caused all of this.