This just occurred to me. On some of my sites I use a facets based search setup so users can drill down to find what they are looking for.
There are 2 issues here; firstly the querystring combinations can be fairly unique and secondly the facets are nofollow / noindex.
I'm seeing quite a few blank ad spaces when looking at these pages and am thinking this may be related to the "Brand Safety" issue.
One thing I was thinking of was editing the URL parameters in Webmaster search console to "Does not effect content". The other is changing the facet links to not be nofollow (which I'd rather not)