Confluence generates attachment xml with <ri:content-entity content-id="..."> that renders as an unknown attachment. When it happens, it affects the whole page, probably multiple pages.
Not known yet.
Attachment is rendered correctly.
Attachment is rendered as an unknown attachment.
Reindex fixes the issue.
Alternatively one can manually remove <ri:content-entity ri:content-id="..."/> inside <ri:attachment> tag using source editor plugin - that also seems to fix it, though much less convenient.