Hi,
It is very likely caused by a 3rd party CSS file. Either another plugin or the theme is loading a stylesheet file on the search plugin back-end, that overrides the styling of those list elements. Usually it is due to bad coding practices, as all plugins should exclusively load their assets on their own pages only, as well as use specific stylesheet rules to prevent such behavior.
If you try disabling all plugins one-by-one and change the theme, at some point it should be fixed. Without log-in details I cannot tell which plugin/theme is at fault there.