There was a post about this bug before.. but the op of that post was not sure if the bug was also with innate DW..
today, ladies and gentlemen, i can confirm that Yes the bug is present even on innate DW.

How the bug is triggered: using a skill. ie Area blast, but being able to kill all the enemies before triggering the next animation will cause lightning to stop/attack at nothing at the cost of MP! Usually mp cost for dual wield is only consumed once.

So watch out fellow ffbe players if you bring a dual wielder into the mix he/she might end up using more MP than usual.

Ways to avoid: you could let the first animation take its time and only act when the second animation has started that way, you avoid OTK the enemies immediately.