I received such a response:
"I am pleased to inform you that the reason for the rejection of the project has been found. The issue was that while you correctly assigned an age rating for the island itself, you did not choose the right one for the island’s image.
According to our specialists, the image for the island (loading screen) should have a rating suitable for viewing by all ages.
In this situation, you will need to set the correct rating for the island’s loading screen, and then you can resubmit the project for review, which should proceed without any problems. After that, your island will be published."