If the values for columns cms_page_id and cmsPage within table product will be set to "NULL", the "original" default layout for product pages will be used, although I've managed to set up a new product-page layout and marked it as "default" beforehand.

Since a different layout was defined as default layout, this particular one should also be used as fallback whenever no layout is assigned.

Steps to reproduce:

1. Create custom product layout
2. Mark it as new default layout for product pages
3. For already existing entries --> set values for mentioned columns to "NULL" or simply remove the layout assignment in the layout-tab of a product
4. clear caches and check the layout used in the storefront

Result: The "original" default layout will still be used as fallback , not the newly created one.

To sum up: It would make sense if the newly created default layout would be used as fallback layout and not the shopware original one.