Yes we are as well, I believe that has to do with how the security in Portal and the security in AGO not matching up. Somehow the security and permissions you've set in portal do not match up with the security and permissions you set up in ArcGIS online. The permissions are copied over from AGO when the dataset is copied into portal. This causes a mismatch in the dataset thus prompting for a username and password. I found that if you allow anonymous access for the dataset, copy it over, and then remove it from AGO, it breaks the link thus allowing you to work with that dataset. Hopefully that makes sense. Please let me know if that works for you.
Thanks
Andrew