Saw a CTR drop today, and went to check.
Looks like it comes from a new design from adsense, their text ads on mobile and tablets are now displayed with an ugly white full-width rectangle at the bottom of the ad (and in that rectangle there's the usual link 'ad choices').
Since my ads have black background, users are sure put away by that whitish rectangle... some will say it decreases invalid clicks, maybe.
But at the end of the day my revenue is getting hit once again, just before christmas.
It's like every six months Adsense comes up with some nasty thing to bug us. End of rant :(