Thank you for the answer. I know what the game instance is, but why do you think it will help?
Even if I save all objects on the map in the game instance, it will break the existing references of the objects on the other map (just tryied that).
For example: If I save the reference of the object_a on the map_a (the reference I need to load variables in this object later) and then save the object_b and object_c on the map_b I will no longer have the reference to the object_a - in the new save file I will have reference to the object_b and c, and the broken (null) reference to the object_a