Cannot select sections which are used in a custom source (if hidden)

[ishetnogferre]

For a client we have a Craft CMS with all kinds of overviews (FAQ, Careers, ...), which are all single sections. For improving the user experience, we've consolidated these overview by using a custom source in the entries section of the CMS and there disabled the separate single sections.

The link field with all entry types are allowed has no problem, but when we want to limit the selectable sections, the custom source doesn't show and so these overview aren't selectable, even when we added them to the selectable list by using their individual items in the list.

All selected (first img) = "Overview" is shown in link field. (second img)

All the overview singles are selected (first) = "Overview" isn't shown (second)