Usually, most compressed games, besides changing the png/jpg files to webp, also does Ctrl+F and changes all the filenames that end in .png to .webp (and video formats to .webm) in the renpy files (the .rpy files) so the compressed "new game" will look for those .webp images instead of the original files.
So unless the game is defined to work only with the filenames without the need to define if it is a .png or a .webp (which is possible but it requires an extra step that I don't know how many devs do) or it already works using only .webp, the "update patch" (which is just some of the old scripts with more code lines and the new images), will overwrite the modifications done by the compresser, thus making the game look for .png files again when those .png files are still .webp.