If you play 2.0 world.. don't cast Heroic Power on mythical attacks. use the hade's spell which is 120 favour. Attack boosters have no effect on mythical units nor do researches.
It's not a bug. It's just a foolish calculation. I've already asked on the Beta forum, you can read the thread here. I definitely agree the spell should be adjusted as it doesn't reduce power of only an attack on which it is casted but... in fact, it also affects attacks which arrive after this. Defender is still at an advantage against attacker.