Do you mean a banner in the header section (at the very top of the page or do you mean just above the forum index? Something to consider, and I've never really thought about this before, is that ads at the very, very top of a page are subject to falling under the dropdown list embedded in the browser itself (in fact, much of all page content falls under this scenario as the page is scrolled).
The top banner is in the "header" section of the code. There are no drop-downs at all in my phpbb pages. Users can click on "view unread posts", etc. but they are well separated from the banner. And the banner is pretty well isolated. It's not impossible for a user to make an accidental click... and with a tablet or smartphone set to the default style it might be easy due to size of fingers. But I don't understand why it would have suddenly started around the first of the year.
The bottom banner is very much isolated from any page controls that I have control over. I think a visitor would have to be a real klutz to make an accidental click on it, quite frankly.
You can tell what units are losing clicks if you're watching performance reports close enough. If your top banner is loosing a lot of clicks and the bottom banner is less subject to this problem, this could be part of the problem. Also, the phpbb main menu (on the main product site) has major dropdowns, if your site is like that and your ads are under the menu, that could also be an issue. And yes I could interpret "top" to mean at the top of your forum index which is under the main menu. So, just trying to get a clearer picture since all we have to work with here are words.
Google scrapes clicks from all the ads (top banner, bottom banner and mobile top). There does not seem to be a pattern and in order to discern a pattern I would have to have a lot more time to hit the refresh button every minute or two.
The bottom banner tends to gather fewer clicks. Right now I have 4 clicks on the top banner, 3 on the bottom banner, and 2 from the mobile.
Remember that on the 27th I had 5 clicks for quite a while, then they were scraped off one at a time... then a couple hours later there were suddenly six clicks... and finally all of THOSE were taken back.
Really, this doesn't look like accidental clicks.