I upgraded from 2018.3.12f to 2019.1.8f and all my Address Assets are gone. Im using latest 1.1.15. Probably need to warn other folks before they upgrade and run into same issue, or put it in documentation.
I had similar troubles with losing a few asset groups each time when upgrading. However, I found out that the group assets were still in Assets/AddressableAssetsData/AssetGroups folder, though they were no longer referenced by the AddressableAssetSettings. I was able to get them back by editing AddressableAssetSettings.asset using version control software. Interestingly, the schema assets for the respective groups were lost and I had to re-add them. Maybe also in your case your group assets could still be there?