Hi!
I opened a support ticket and just received an answer. James’ reply was:
Can you please deactivate and re-activate your ACF PRO license (from the custom fields -> updates page) and try the plugin update again?
You can also try the ‘Dashboard -> Updates’ page and click the ‘Check again’ button to clear plugin update cached data.
The first solution did the trick for me.
Hope it works for you 🙂