It is because the sidebar form has its own search box, which is hidden. It retains the text search from when you arrive on the page.

You have a search at the top of the page from the theme, which you may have coded to send to Search & Filter using a rewrite?

Then you have another form above the results, and the same form repeated in the sidebar. These two forms will not ‘talk to each other.

In V3 of our plugin we intend to allow you to place different parts of the form in different locations on your page, which is what you need, but for now you must have the search form in ONE of these locations, but not both.