Using the file browser causes a thick white border to appear around the search bar, and it doesn't go away unless you cycle through several different themes or restart the app.
The following recording shows this in action. Notice how the border appears as soon as a file preview thumbnail appears.
At the end, you can see that the search bar now has a thick border around it.
Alfred version & build number - v3.5 [876]
OS X version - High Sierra 10.13 (17A365)