For games that are already posted to the store, any versions made and not yet published will not authorize new accounts to access Google Play Games for the game. The workaround is to use the account to download the Live build to authorize it; then that account can be used with test builds.
Ads between matches WERE working, as was the purchase option.
This was found in Shipping, but I was able to reproduce and gather logs from Development.
This was found on a Pixel 2 on Android 8.1
RESULT: Achievements and Leaderboards are grayed out
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-54902 in the post.