I use Word 2016 for Mac. I'm adding a watermark and within 'Size and Position/Advanced Layout' I'm fixing the watermark so that it is aligned to the page and not the margin. I'm doing this so that it stays fixed in the background when my client starts to adjust margins.

After saving and closing the document, I then reopen it, and the watermark has moved.
I check the settings and I can see that Alignment has changed fromPage back to Margin.

Why is this? Is this a bug in Word?