How to make SEF URLs?

I do have another content type, Suppliers, although we do not show the info page for this on the front end. But if I go to a Supplier and click Preview the URL is:
https://my-site/product/root/uncategorised/bobs-tvs?preview=1&tmpl=component

But this breaks styling, I have to remove the ?preview=1&tmpl=component from the end to view the page styled:
https://my-site/product/root/uncategorised/bobs-tvs

This also works to show the content and you can see it has /root/ in it.

I have not configured suppliers to have /suppliers/ in the URL as we do not need to show these pages, so its taking /product/ as default.

If I remove root, its a 404.

I have fixed it!!!

Your comment "root is the alias of the top level menu item with ID 1 , and it should not appear (regardless of configuration)" made me think about the following...

I went to Menus -> Flexicontent Hidden Menu, and I did not create a NEW menu for products in here, but i edited the one that was there, as I only need one Flexicontent hidden menu for products.

So when you said root was for the default, I thought maybe its something to do with the settings on the hidden menu that was there, that I changed.

So I created a NEW flexicontent hidden menu, assigned it to Product Type, and it worked.

I tried cross referencing settings between the them but cant see any difference, so am not sure if there is a setting in the original default one to add root?