Hello,
Please check the issue on one of the default themes to understand if our theme causes the problem.
Most likely the issue is in multilingual configurations. Make sure you have added translations to all the products, search the product in the language you want to get search results for. Besides, take into consideration, that the search finds the product by a keyword, so if the keyword is the same for all languages (for example a brand) you will get the results from all the languages
Best Regards