I'm not sure if any of the other sections have the same issue, but I have several resources I hid that show up in the "Related Books" section of Insights:
In this example, I've hidden the Portuguese titles listed at the bottom of the dropdown.
Can you also show a screen shot of the hidden list in Program settings so we can see that they are hidden?
I should have thought about making sure I could prove it. :) I picked one of them here and clicked on it. You can see it even shows the content in the "Related books" section but is clearly marked hidden when I click on it. I can hunt it down in Program settings if needed, but I imagine this is sufficient.