Pages 15.3.1: DOCX import changes the header from three fields to a single field

When I create a new document in Pages 15.3.1, the header correctly shows three separate fields (Left, Centre, and Right). However, when I open/import a DOCX file, the header immediately appears as a single continuous field instead of the three separate fields.

The DOCX file itself does not contain a header or footer. In Pages, Header is enabled and the document is a word-processing document. Even after enabling Facing Pages and “Left and right pages are different,” the imported DOCX still shows the header as a single field.

Is this a DOCX import/conversion issue or a bug in Pages 15.3.1? Is there a way to convert the imported header into the native three-field Pages header structure?