I have the same problem on xf 1.5 for some time on the find-new, search and conversation pages (all ads). The adsense code is on the pages correct so i am thinking this is something from google. Before they did show.
Changing it to an other banner code does not work. I also have google dfp for showing rotating sponsor ad banners and they do show.
So adsense does not want to show ads on those pages for some reason?
1 thing i could find is that adsense is showing on my popular content but not top posters. First one can be accesed if you got the link or are logged is, second one only when logged in. Adsense has access to crawl al my pages.
https://forum.bodybuilding.nl/find-popular/content/
https://forum.bodybuilding.nl/top-users/
Conversations i do not now how adsense access it, find-new has nofollow+noindex on them and search results has noindex. I did read google does not crawl pages anymore with noindex after some time and this is then the same as noindex, nofollow. But adsense does not use googlebot?
I also did a test with media pages. I had set albums to only show for members and after some time google did not show any ads on them. The code was still correct on the page and adsense should have acces to them. Now guest can see albums again and adsense is comming back for some. Then i blocked them in robots.txt (googlebot only) and not showing again (mediapartners allowed).
I did the same for some media catagories that are not blocked in robots.txt or noindex/nofollow and they are showing, So i am thinking it has something to do with what adsense crawls and what we set for googlebot has effect on it? This is recent so must be a change or fault from there side.