In addition to the bug report in issue UE-80948 :
When renaming or moving a custom GameMode BP or GameInstance BP, the values in the DefaultEngine.ini are not updated. Running in the editor will work without problems, but a packaged game will not find the old assets and default back to the default GameMode and/or Instance.
(if there is a way to add this extra info to the bug report, please let me know as I could not find this option)