Probably not a bug but a semi-obscure feature [:)]
This army would have to pay only 1 to move into a hilly hex. It should be 2. And what I suspect is that given it has attacked, there is a special allowance to move into a hex for one less ...
I seem to recall a previous report whereby a unit that had attacked could move in this way providing the user had not unclicked control. But as soon as they did so, the unit could no longer move.