Lunar Blessing not receiving correct duration from Skill Effect Duration

Lunar Blessing has a base duration of 5 with .2 seconds added per Rage spent to cast it.

With my max rage, it ends up at a 15 second duration, but with Prolonged Duration II socketed into it, it only goes up to a 17 second duration. 32% Skill Effect Duration from the passive tree also only gives 2 extra seconds.

Skill Effect Duration is only adding on from the base 5 second duration instead of the total duration after Rage is factored in.