If you are wondering why some cover images you set are not be showing as cover images in prestashop 1.7, then this article will help you figure out the reason.
How to set product cover images in pretashop 1.7
I think that's not a bug, but I am not sure that's the right way, I have created a tool to force prestashop 1.7 to show cover images when they are not images assigned to the default combination, but it works for product listing pages only, to make the tool work for products on the homepage requires to modify some files, that's not cool. I will try to find a way to force prestashop to display cover images without modification any file.
Current when you go to set a cover image for a product, choose one image which is assigned to the default combination to be the product's cover image.