SOLUTION: (Thanks to the Appflow support team)
For reference, this issue was resolved in the following way:
I removed the following line from the build.gradle file:
archivesBaseName = "$applicationId-v$versionCode($versionName)"
That entry changes the builds output .apk name, in order to make the buildsystem produce a separate apk for each version. (Which is pretty convenient btw, when building locally)
I manually added that entry a long time ago, and never thought about it again.