Hello!
I have never been able to re-create any errors with quick save and have not be able to find much information on this specific error. It's possible that this is an issue within the Ren'py engine's core code (which I did not build and cannot fix) and the way it is storing save data on your machine.
Does this problem persist if you create a manual save? And does the problem persist if you close out of the game and restart? I'm not sure what could specifically be going on here.
"Quick save" and how it works is basically a native Ren'py function that I didn't change in any way with my game code. It can make errors like this really difficult to track down, much less fix.
If you are able to manually save, then I would stick with that for now and I will continue to keep my eye out for this error but so far, I haven't been able to recreate it so I can't say what is causing it. ;A;