My big question: Did you click into the InDesign Articles panel menu button and turn on “Use for Tagging Order in Tagged PDF”?
The word Article gets used in various ways in Adobe software.
https://helpx.adobe.com/in/acrobat/using/pdf-articles.html
In Adobe Acrobat Pro, the Articles panel is used to define, navigate, and manage paths of continuous text that span across multiple columns or pages. It allows sighted readers to follow a specific story or article sequentially without getting visually lost in complex newsletter, newspaper, or magazine-style layouts. The Articles panel is for guided reading for people who can see. When reading a document, clicking on a defined article lets you view the text sequentially. The software automatically zooms and pans from one text box (article box) to the next in the order you set.
This is not the same as accessibility for speech readers for persons who cannot see.
The Articles panel in InDesign is very different. It helps you decide what will go into an exported PDF or EPUB. Objects not in the InDesign Articles panel will not be in the Accessibility tags structure of the exported PDF and so will not be read out loud by an accessibility software.
An <Art> tag in the Acrobat Accessibility Tags panel stands for Article. It acts as a structural container for a self-contained, independent body of text, like a magazine article, newspaper story, or blog post. Screen readers and assistive technologies use it to understand and navigate logical document boundaries. There are 38 tags to get to know in Acrobat PDF Accessibility.
https://www.section508.gov/create/pdfs/common-tags-and-usage/
I hope this helps.