Hi K,
I have had some issues for a while with those rendered files building up.
The app tries to read the wallpaper database to figure out which ones are in use (you probably got a permission request for that) - but it's not working great.
various things trigger re-rendering, particularly settings to reapply when you change spaces
I haven't had time to sort this out for a while, so I don't know when it will be right.
sorry