Hold up, the backer .apk for every other version has been roughly 680+ megabytes due to having higher quality images. This one (and the previous one you posted) is only 136 mb, which is the same size as the current public .apk. If the devs accidentally published the public build as a backer build, that could explain why people with previous backer .apks are having issues updating. The signatures might be different or something. Could you double check to make sure that the current backer download link hasn't been silently updated or something to fix that?
I dug a bit more into this and did confirm that the x2 and x4 resolution images are not present at all in the .apk posted. It
does seem to be a backer version based on my limited exploration of the game files and code, but I have no idea if the devs have intentionally removed the higher quality images and this is how their dev builds just
are now, or if they fucked up somewhere. Hoping for the latter, since if this is just how dev builds
are now, idk how updating the android app will work since they seem to have fucked up and made it not be recognized as a legitimate update or something, causing the update to fail.
As a sidenote, in the .apk archive there's corresponding .map files for all the game's .js files, which I
assume are the sourcemaps that are hidden/not present in the windows builds. I know I've seen some people asking about getting those so you
may be able to get them from the .apk archive. IDK anything about that stuff tho so don't yell at me if I'm wrong