Currently this is by design, but the XF1 version support matching ALL or matching ANY. That feature hasn't been ported to the XF2 version yet.
The main idea is you can click on a prefix, and it gets added to the filter, then click on another prefix to add it as well to narrow the search. This is how the other filter options tend to work, and is how it is commonly done on other sites & systems.
I could work on the 'any' prefix feature and workout how to expose that to the filtering UI, but it will be sometime before I can do that design work