MongoDB | Problem inserting embedded documents into document
To solve this problem I changed the code to the following:

lt_mode_data = data['lifetime']['mode']
    mode_dict = {}
    game_mode_data = GameModeData()
    lifetime_mode = LifetimeMode()
    for index, (mode, mode_data) in enumerate(lt_mode_data.items()):
        mode_dict[mode] = mode_data['properties']
    lifetime_mode = lifetime_mode.from_json(game_mode_data.from_json(json.dumps(mode_dict)).to_json())