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?