Looking at the issue in service desk it says that it was approved right above the list of approvers, but the issue didn't transition out of 'waiting for approval'.
Clicking on the approve button while looking at the issue in service desk DOES transition to the next state.
So it seems that somehow the approve operation in the portal isn't triggering the state transition in the workflow but service desk records the fact that it is approved.
What do I need to do to get the transition to also happen?