When I search for content in the portlet, the right content is shown.
But when I try to limit search or sort search results, all content of the site is shown.
If I search on the search form, everything is also shown.
I can see now javascript errors. I have lineage installed.
Update: I added a new Plone site without lineage and it has the same problem
You don't say whether you're testing anonymously or what roles the logged in user has. Does this happen on a plain vanilla Plone 5.0.6 site or one that has lots of add-ons?