Filing this as maybe-a-bug, because of UX inconsistencies.
When hyperlinks appear in the first line of a Category’s ‘About Category’ page, they are displayed as such in the brief category descriptions on the All Categories page, and in subcategory boxes (if those are selected for display in settings).
Upon drilling down into a category and finding the ‘About Category’ topic pinned, the same first-line text is displayed (‘preview text’?), but without hyperlinks. Sometimes however, no first-line text is displayed, but only the name of the ‘About Category’ topic’s parent category repeated (in sub-category style, preceded by a bullet/box). I can’t quite work out the pattern here.