Short answer
HubSpot's KB: compare the page source with HubSpot's embed code and re-embed if they differ. If they match, other HTML or JavaScript may interfere. Add the external page's domain as an additional site domain, or submissions are marked as spam.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
The KB also covers WordPress-specific issues.
3. Related
Additional site domains and spam submissions.