@hwk
Got around to setting up a test site 🙂
I am not using have_archive – loop.
I enable debug mode and can confirm that there is data in the field on the archive page.
I wonder if ACF sees the archive page as an option page that is outside acf scope and that I therefore should use the wordpress get options function instead?
I can give you access to the site if you want. Just send me a mail 🙂