Potential Bug : User.ID not masquerading properly

When logging into a Community as a User via the Contact record the {!$User.Id} token contained within a Visualforce page contained withinan a Home Page Component displayed via an iFrame it shows my ID rather than the ID of the person I am masquerading as.

Example of the Component code

<iframe src="https://c.na6.visual.force.com/apex/TBR_App_Reviewer_Portal_HP?core.apexpages.devmode.url=1" width="98%" frameborder="0" height="400"></iframe>

When accessing a legacy portal the ID shows as the user I am masquerading as..
When accessomg a community the ID shows my user ID

Is this a bug?