I have tried the default 2016 theme and search works as expected.
Both sites using Basel have the same issue with the search feature though. Upon further investigation, I have discovered that the problem is actually with the autocomplete results. The search is performed correctly if I click the “Search” button or the “Enter” key, but the autocomplete drop-down shows the results only in the main site language.
As you can see in the screenshots, even though I am in the English version of the site and I look for a product in English I get results in French in the drop-down. If I click on the results provided in the drop-down, it takes me to the French product.