Sounds like a really good reason not to use Obtainium, if any repo you have tracked for updates can just redirect you to a completely different repo If they have the keys - and throw no complaints when updating to an entirely different apk.

With F-Droid they at least have to have the same signing keys, and the code must be a replicable build by F-Droid’s internal apk signature copying process - meaning the code for the supplied APK always matches the code on the repository for the build.