Figured I’d keep the thread updated in case anyone else runs into this issue. I created a new blank blueprint project, and added **only **the basic login and leaderboard functionality (plus a dummy score counter). Setup a new app on Google Play, uploaded the APK, and it works perfectly.
So I’m thinking there’s either something not right with the settings on the Google Play side, or a config file got broken somehow. If it’s the latter, at this point it’ll probably just be easier to create a fresh project and migrate everything across.