That's... impressive work!
I want to ask if you've already translated the meaning of the numbers? We have letter clues, like 18=E, 83=H, and so on, from investigating the locations. Because of this, when you correctly substitute all those numbers and think about it a little, they form the sentence: 'You are the creator.' If yes, what do u think about it?
so,
this is just my theory, but most likely, this cipher was created by the mc before she lost her memory (since this cipher and these numbers are written on her paintings with her name on them, except for the mug). She probably created it for us - it suggests mc knew something was going to happen.
From there, you can start developing your own theories, and I also lean towards the idea that the main character is dead, or something similar..