I noticed that the title still changes to some API Portal defaults when navigating e.g. to the API gallery. This can be solved by adjusting additional properties. The relevant file is <InstallDir>/API_Portal/server/bin/work/work_apiportalbundle_<size>/base/webapps/ROOT/WEB-INF/config/apiportal/localization/views/views.properties and the properties to be changed are views.gallery.browserTabTitle.DBI etc., i. e. all properties that contain the name browserTabTitle.