Hi Kevin,
My apologies for the delay. Upon discussing with the rest of our team, it appears that the field you specified is intended for a text ad, not really for a shopping product ad, so it's expected that nothing is returned. Shopping product ads, by its nature, probably doesn't contain any headline fields that the user in the UI can modify by themselves directly.
The Headline field in the AdWords API is probably populated automatically somehow but that part would be the same for all shopping product ads. With that, it would appear that the Google Ads API does not support returning the fixed headline anymore.
Thanks and regards,