Looks like I have the same issue 10 months down the line using Acrobat Standard DC I added the Watermark to the existing PDF with the Apperance Options set to only "Show when Printing". This worked a treat!
After I sent the document out for signing (Adobe Sign), the resultant signed PDF now displays the Watermark on the Screen. The signed document is locked for editing, so I'm unable to change the watermark settings, but they don't report to have changed anyway!
So far, I haven't found a solution, or work around, so I'd welcome any feedback.