Linked Variations: Unable to add products – Incorrect list

I have successfully created some linkable variations. Now that my product list is larger, it’s impossible to add the products in the variations because the search doesn’t work. If I write a word, it doesn’t show the correct results, but only the list of products in loading order, and in limited number. For example, in the attached screen I was looking for the word “Nero” but it shows me results where that word is not there. I have uploaded several products with the word “Nero” in the title. It doesn’t even work if I write the product name entirely. I need to fix this urgently, thank you.