Finale of Devastation (or Blunale, as I think of it) is supposed to give drawn cards mana if it has more than 6 mana stored. However, I’ve had several instances this event where it has had between 6-9 mana and not given any mana to the cards drawn.
Its working as intended, its just written REALLY badly.
I explained it here:
https://forums.505go.com/discussion/79463/finale-of-revelation-doesnt-give-extra-mana-since-3-7