At present, Next Page Number and Previous Page Number actually means Next Text Frame Page Number / Previous Text Frame Page Number. It is very confusing. I suggest to add real Next Page Number / Previous Page Number which would add – surprise! – next page number and previous page number to the text.
It would be very helpful, in situations you want to have page numbers only at one page of a spread (10/11, 12–13 etc). At the moment, there is only a stupid and slow workaround with linked text frames, which does not work in every situations.