Okay – I recall someone a few months ago had a very similar issue. Then we found, that he was using the wrong widget, as he wanted to filter an archive page – which is not possible.
Make sure, that you are using the search in combination with the Elementor Posts Widget (not the Posts Archive widget), and not on an Archive page (like category, taxonomy, tag etc.. archives, except for the actual search results page) via a custom template of some sort. Archive pages are very problematic, and will not work for most cases.