I think I know what the bug is being caused by, and unfortunately it’s not something that can be fixed on your end! It has to do with how the game is saved/a limit on the amount of data one save can contain (so in your case, past a certain point the game decides the data's too big to save anymore; I have no clue why this is happening to you and not other players though).
I’m going to look into changing how my saves work while not losing everyone’s saves, but that’ll take quite some time. I'll work on that along with the Chapter 4 update!
Thank you for reporting this, and sorry for the trouble!