I have looked, and the option to highlight instead of restricting the book list to the results is never checked. But when I open Calibre, it highlights every time. I have to go in to the Search options, check and then uncheck the highlight option, and then click Apply. Then it works like I want it to without having to do that again until I have closed and reopened Calibre the next time. Why??

It didn't used to do that.
Edit to add: Using Calibre 8.5 on the latest Windows.