Create a portfolio in the Portfolios tool.
In the Add/Edit Content tab of the Portfolios tool next to a form, either select an existing form or click 'Create New' to create and select a new form
Click 'Preview the Portfolio' – The new form is not displayed
Click 'Save', then 'Preview the Portfolio' – now the new form is correctly displayed
I have attached a simple matrix and portfolio template that display the problem, though is should work the same in any portfolio.