This is working as designed because Universes are supposed to be narrowed views and is a step towards them being used as a security/access control feature.
To add more attribute values requires deleting the existing attribute filter and adding it back with the required values.
Currently in the latest release 10.5.x if delete the attribute filter and add it back in the same session the attribute value list is still not refreshed to show the full list. To workaround that save the Universe after deleting the filter and edit the Universe again to add the filter which will then show the full list of attribute values. This problem will be fixed in future APM release 10.6