It depends on the order in which you assign the Modifiers. Extra Phase is only valid if the Power is Persistent, until then, the lowest valid value is 1 Turn. Not too much different from Time Limit only becoming available after Reduced Endurance is assigned.
If you assign Time Limit before assigning Persistent, then you'll see 1 Turn as the first option in the drop down. If you then assign Persistent and go back in to edit Time Limit, you'll find that Extra Phase is available.
Alternately, if you assign Persistent before assigning Time Limit, Extra Phase will be available from the start.