sounds more like an intentional backdoor
Not really, this type of save changes at the end used to be fairly common (i assume for performance reasons on big docs back when computers were much more constrained) microsoft word did the same thing back in the day.
It's not only common it's still the way PDFs are usually saved. Open a PDF in a text editor (PDFs are text not binary files) and you can see any edits appended as "trailers".
This sounds like a feature that could be exploited in creative ways in either a product or some fun side project. I don't know what it is exactly but there's something aout never editing edit logs (possible not being obvious to the user as a factor) plus some graphical UI representation or UX flow (besides undo).
There’s always a scary world lurking underneath it seems.