I was thinking maybe there was a plugin allowing you to include the custom field data towards the end of each post, just like tags are optional to include in the feed with the UltimateTagWarrior plugin.
Are you using some code to display the 468 ads in the first posts?
If so, this is probably the problem.
I used to have a code for only diplaying a 468 ad after the first post, this made the tower in the sidebar disappear. I then decided to only go for the tower in the sidebar. Here’s how it looks: https://www.smidigt.se/