InDesign treats Parent Page items as background structure, almost like a watermark or a template layer. While these items are visible to the eye, the Articles panel can only interact with objects that exist directly on the document page itself. This is why headings or images on a Parent Page won't show up in your reading order or export correctly to a tagged PDF unless you intervene.
To fix this, you have to "bring the items to life" on the document page by overriding them. By holding Cmd/Ctrl + Shift and clicking the item, you transform it into a local object. Once it is a local object, the Articles panel can finally recognize it, allowing you to drag it into the logical reading sequence.
While you could technically rely on InDesign’s auto-tagging during export, that method is notoriously unreliable and often scrambles the reading order. For a truly accessible document, the best approach is to keep decorative "items" on the Parent Page and move all semantic content like titles and meaningful images onto the document pages. Manually building your Articles panel this way is the should ensure a screen reader follows your layout exactly as intended.