When I select alternate/showcase card styles for my deck image, the art reverts to vanilla after saving. This happens with any alternate card style, e.g.,

-Throne of Eldraine: adventure showcase card styles
-Ikoria: mutate showcase card styles
-Theros: God/demigod showcase card styles

These alternate/showcase styles are sought after because they are rare and usually not free. I would prefer to get the most use out them as a reward for obtaining them. That includes using them for my deck images.