Hi ronaldk, I am having the exact same issue with my game but with both leaderboards and achievements.
I would love some more details on how you fixed it and whether you could help me out at all.
I have made a post in the Unreal Forums but no-one has responded with any direction yet.
https://forums.unrealengine.com/development-discussion/ios-development/1526700-gamecenter-leaderboards-and-achievements
When you grouped the leaderboards on connect it adds grp. at the beginning. Did you add grp. to the leaderboard name in the Android settings as well as the blueprint nodes after grouping?
Are you certain this is what solved your issue at all? Also, were you getting any scores being posted to your leaderbords with builds on testflight, or do they only post scores after the build is ready for sale in the app store?