This unfortunately is a limitation with Netdocuments (ND) stamp setting "footer every page" - it will always overwrite your document's footer on the first page and places the document ID in it's place.
You can set ND settings to the option "footer every page, except first" but if the document is only 1 page then you won't get any document ID appear on the first page.
As ND document saving is very streamlined and simplified we have suggested to many clients that perhaps the document ID is a requirement you may not need because the search in ND is so powerful that locating the document by various other forms (i.e by searching for words contained inside a document) does the same thing.
To access the setting "footer every page, except first" got to the nd icon at bottom right hand of your screen
Click on the cog and choose settings, ID stamp, footer every page, except first and click OK.