Prefabs already in the scene when entering Play Mode are unpacked, but Prefabs dragged in while in Play Mode are not. This causes issue such as scripts that reparent in Awake causing errors that the Prefab instance can't be modified. Runtime logic should never cause these editor workflow specific errors. Fix this by unpacking Prefabs when they are dragged into the scene in Play Mode. Should only happen in Play Mode, and only if the destination scene is not a Preview scene. Must happen prior to all script callbacks, such as Awake.