In a document with both portrait and landscape sections, there will be section breaks, and each section contains at least one header (and footer). By default, the headers (and footers) are linked, that is, they have the same content. A table whose width is set to 100% appears to expand to fill the space between the margins (in the header) in the portrait section, but it won't do the same in the landscape section.
In other words, 100% width is determined by the header width in the portrait section.

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Stefan Blom
Microsoft Word MVP

Microsoft 365 apps for business
Windows 11 Professional