ACF fields are not save during an autosave. Autosave is a core WP feature and no custom fields are saved or sent to the server when it is run. As far as I know there are not solutions to this, at least I have not been able to find any. see also https://support.advancedcustomfields.com/forums/topic/saving-acf-properties-when-wp-runs-auto-save-2/
