On try to export page from history as PDF Confluence gives current version.
How I reproduced:
1) created new page
2) edited content
3) opened Info->View page history and clicked on first version of document
4) Clickin on 'export page to pdf' gives current version (but not selected)

Workaround
Restore the historical version that you would like to export, export it and restore the current version.
You can delete these restores if you do not want it cluttering your history.