The problem was simply a missing configuration 🙂
On the Search instance general options panel the “Index table engine” was not selected, nor the option to search within products: https://i.imgur.com/8dvlt2k.png
Individual search instances need to be configured independently from the index table configuration – as they can have different configurations as the index table itself.
I've selected the product custom post type and the correct engine, now it should display results correctly.Best,
If you like my products, don't forget to rate them on codecanyon :)