Reproduction steps:
1. Open the attached project "AddressablesBug.zip"
2. Delete old AddressableAssetsData folder
3. Create AddressableAssetSettings
4. If "Legacy Bundles Detected" window pops up, click Ignore
4. Create MyGroup and assign assets from DLC folder to this group (Cube1, Image1, Material1)
5. Press "Simplify Addressables Names" on each asset
6. Set the Asset Bundle Provider property to Custom AssetBundle Provider for MyGroup
7. Build -> New Build -> Default Build Script
8. Press Play and click the button on the screen

Expected result: The cube is loaded in the Scene
Actual result: The cube does not load, these errors pop up:
"ArgumentException: The Object you want to instantiate is null." 'UnityEngine.ResourceManagement.Exceptions.UnknownResourceProviderException' was thrown., ProviderId=UnityEngine.ResourceManagement.ResourceProviders.AssetBundleProvider "

Reproducible with: 1.14.2 - 1.17.17 (2019.4.25f1, 2020.3.7f1, 2021.1.8f1, 2021.2.0a17)