It works. Try it. I know it isn't what you want. It is the best I can come up with. I do not understand the +1 either, but it does display on the last page. It will always display on the last page and in the instances where currently it goes missing, it will display on the last page and the next-to-last page.

The problem is that your footer changes the page size so that if it is not showing, the page is the last page. If it is showing, it is not the last page because it changes the pagination and thus doesn't meet the condition. I'm somewhat surprised it doesn't crash Word when it goes into this state.

You could make your table smaller so that it would always fit in the footer area and not change the space available for normal text.

Paul Edstein is a regular contributor here and elsewhere and the master of Word fields. He is a Microsoft Word MVP.
Look at some of the pinned threads on fields including this one. I expect he will look at this before long.