Word 2016 - continuous page numbering resets after saving

Question

I have a small Word document. All pages are in portrait except pages 8..10.

In the footer I've inserted page numbering, that continuously counts from one page to the next. This works well, Except when it gets to the first landscape-formatted page, page 8, where it resets to 0, and then again when the orientation becomes portrait again.

The page numbering becomes 1, 2, 3, ... 7, 0, 1, 2, 0, 1, 2, ...

Right-clicking on the page number, format page number, continue from previous page number will fix the problem, but only temporarily: I save the document, but the page numbering is failing when I re-open the document!

Any suggestions for fixing this?

UPDATE! After some more research, I discovered that this "feature" is somehow related to my Header 3 typography. Using 'Normal', I can select “Layout” or “Page Layout” > “Breaks” > “Next Page” to create a section. As soon as I select the line right after the "Next Page" break, set it to Header 3, the page numbering resets after save/open, and the format cannot be made workable again, until I delete the lines containing the "Next Page" break.

Question now is: What in the Header 3 can make "Next Page" breaks behave like this?

Another update When opening the document, the pages in the index appears to be fine, until the problematic page number has been displayed. The document can be printed with correct page numbers, but once I display the page number on the screen, it resets to 0 (the index updates the page number to 0 and the page number shows 0). Closing the document will not register it as being changed - it will not ask if the document should be saved.