Hi, I just confirmed that I see the same thing on Mac Joplin 2.4.5 (prod, darwin)
If I click the search box, and then delete all the characters, the search box loses focus. It puts the application back to the last notebook that I was in.
I have a feeling this is because without anything in the search box there is no longer a list of notes to display and so it goes back where you were.
I am able to work around this issue by using Command+Shift+F to bring the search box back into focus without needing to click.
For what it's worth, I feel like your description was clear in English. You can probably submit as is, but it always helps to write out what you want to happen (and obviously the devs will choose whether it is worth fixing or causes other issues).