Hello Bayard,
Thank you for reaching out to us.
In general, when editing or previewing a page using the SP Page Builder front-end editor, the URL will display using the page ID rather than the alias. This is expected behavior during edit and preview modes.
However, In edit mode header is ok but in preview mode header is overlapped with page section and It's seems here different header is assigned.
In main menu page, If it's not create this issues, I think you can ignore it.
Is there anything else I can assist you with?
Best regards,