I can confirm this behavior. It appears that Word doesn't accept a header that large. Instead, use a text box, possibly anchored to the first page header.

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Stefan Blom
Microsoft Word MVP

Microsoft 365 apps for business
Windows 11 Professional

Thanks Stefan, I will try that and let you know how it works. If this is definite behaviour then it may cause us problems - integrating some output from a third party system into a Word template.

Fingers crossed that your suggestion works!