Hi Zoltan,
Thank you for raising this concern to the Google Ads API Forum.
With regard to your concern "It's possible to set an optional call-to-action and headline for bumper ads via the Google Ads UI and Google Ads editor "that you mentioned. I can see that this is out of our scope already. With this, I suggest clarifying this to
Google Ads Help and
Google Ads Editor Help by describing your issues / concerns. Regarding the supported Ad types in reports via Google Ads API, ad_group_ad.ad.type bumper ad is not supported. That said, could you please confirm if you are able to see the bumper ad in the Google Ads UI? If yes, then provide a screenshot of the Google Ads UI so that we can raise a feature request, subject for review.
Best regards,
Jinky
Google Ads API Team