I have the same issue. Its extremely annoying as I maximize Unity tabs all the time.
The problem appears after PM window is restored to docked state from maximized state. If PM tab/window is closed and then reopened it works fine until its demaximized again.
The actual problem is that PM editor is not changing to the currently selected gameObject FSM - i.e. "Select Game Object" dropdown (Number 2 on the Selection Toolbar screenshot) doesn't work (it actually changes the game object, but the FSM is not changed), same problem if gameObject is changed through standard hierarchy selection.
As OP said it can be forced to change to the active gameObject's FSM by (1) digging in the hierarchy to find the GO and click on the Edit button on the component, or (2) by selecting the Recent button (top left on the PM window) and hoping for the desired FSM to be in the list.