As per my understand I think default Magento work like it's show all images in bottom images gallery and it will update only in main image as per your selection. for example If you select red option then main image show red option image.
If none of the other solutions are working, check your database table inventory_source_item. If all the products have a status of 0 then update them to 1 and see if that helps.
UPDATE `inventory_source_item` SET `status` = '1' WHERE `inventory_source_item`.`status` = 0;