How to reproduce:
1. Open the attached "project.zip" Unity project
2. Log in to the Game Center in your iOS device
3. Build and deploy the "SampleScene" Scene
4. Tap the button on the screen and observe the Xcode console

Expected result: the GameCenterUsersID is printed in the console
Actual result: "UnknownID" is printed in the console

Reproducible with: 2019.4.2f1, 2019.4.8f1, 2020.1.0a2, 2020.1.2f1, 2020.2.0a20
Not reproducible with: 2018.4.26f1, 2019.3.16f1, 2019.4.1f1, 2020.1.0a1

Reproduces on:
VLNQA00160 iPhone 6S iOS 13.3.1

Notes:
1. If the "Player is unauthenticated" message is received, make sure you are logged in to the Game Center on your device.