When you have selected patron attribute to be searchable in staff patron searches, this can only be done in the patrons page. However there are many other pages that have the patron search box available where you can't search with the extended patron attributes. I think patron attribute search should be available always when patron search is available. It would be more consistent with the description of the preference. "Check to make this attribute searchable in staff patron searches. If checked, this attribute will appear in patron search dropdowns."
I do think this can be fixed by adding [% USE ExtendedAttributeTypes %] to member-search-box.inc. I did try this fix and it seemed to work but I am not a coder and haven't read through the coding guidelines yet so maybe this isn't the preferred way of doing things.