Numbers - wrong (old) header in print preview

I have a Numbers document that I use for our family budget. I duplicate it each month and then change the headers to reflect the month and the year. I've been using it since 2008. I just discovered that when I go to create a pdf for my DropBox archive, it changes the header to December 2020 while in print preview (and also prints it as December 2020). It's been doing this for at least a couple of years, but I didn't notice it until recently. I have no idea how to solve this. Help! Thank you!