Adding watermark to Word document apparently corrupts layout

Question

I have a Microsoft Word for Mac 15.15 document with pages whose entire space is taken up by equivalent 3 x 3 tables. Its orientation is landscape.

When I try to add a watermark to this document its layout is corrupted in that 3rd rows move to subsequent pages. What could explain this and is there anything I can do about it? (E.g. does adding a watermark have any implicit effect on the size of headers/footers, say?)