I agree that maintaining it's shape is not the baseline for a protean. However, the ability to do so is on the stat sheet of the protean itself. So, it could realistically be able to perform this action if it desires, even if it does not know it is a protean.
Where-as the abilities it is copying are not native to the protean. So it maybe doesn't understand what they are, how to activate them, or even that it has them.