When you activate "Different first page" it applies to both headers and footers of the section. Any content that should be the same in first page and main headers (or footers) you will have to add manually.

If you want to repeat content in a specific style, such as a heading, you can make use of a STYLEREF field. Post back if you need details.

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Stefan Blom
Microsoft Word MVP

Microsoft 365 apps for business
Windows 11 Professional