Same thing happend to me, I re-did the whole story-line in order.
I just finish all my quest in mythsong saga but when i enter in intochaos warlic say " mythsong saga is incomplete ".
But when i check in my book of lore all my cutscene is complete.
imgur: the simple image sharer
imgur: the simple image sharer
imgur: the simple image sharer
Same thing happend to me, I re-did the whole story-line in order.
It's a bug in the game. Here's a thread about it on the official BattleOn forum.