I have set a few pages on the power pages portal to be only readable by users with certain web roles.
I have a portal contact who does not have those web roles.
The contact can always read the pages in question regardless of what web roles he does or doesn't have, or even if he has no web roles whatsoever.
On a side note, if I go to the web roles tab while managing the contact, I can't find a single web role when trying to assign them a web role. However, I can manage the web roles and add them to the contact through there, after which they do show up in the web roles tab of the contact. Unsure why this is, or if it is relevant to the roles not working correctly in the first place.
