To reproduce add a border to a paragraph style (not the frame, the paragraph) apply this to an existing paragraph and export to PDF. In Acrobat use preflight to check for pdf/ua compliance. You will get an error message telling there is a content that is neither an artifact nor a tagged content.

To fix this in Acrobat, you have to select the "path" elements in navigation pane and apply an apropriate parent container. Reason for this behavior is, that InDesign places the path that renders the border directly in the structure without a container element. And this does not conform to pdf/ua.

I added two files to this post. One with the error message in preflight, the second shows a textfield of a demo form, where the labels is created with a paragraph style with a top boarder line. The top-border line is then created as path without parent in the PDF.