How to reproduce:
1. Open attached "addressables-experiments.zip" project and Scenes/DownloadSizeTests scene
2. Enter the Play mode
3. Observe the errors in the Console window

Expected result: No errors in the Console window
Actual result: Errors in the Console window appear

Reproducible with: 1.16.1 - 1.16.7(2018.4.28f1, 2019.4.14f1, 2020.1.12f1, 2020.2.0b11, 2021.1.0a4)
Not reproducible with: 1.10.0 - 1.15.1(2018.4.28f1, 2019.4.14f1, 2020.1.12f1, 2020.2.0b11, 2021.1.0a4)

Notes:
1. GetDownloadSizeAsync() works as expected for the "Simulate Groups" and "Use Existing Build" play mode options.
2. The exception that is shown in the Console window:

"Failed to get download size: System.Exception: ChainOperation of Type: System.Int64 failed because dependent operation failed
Exception of type 'UnityEngine.AddressableAssets.InvalidKeyException' was thrown., Key=CancelLoadTest, Type=System.Object"