In MC-268812, most backgrounds that previously didn't had the dirt background were reverted to not having it in the Programmer art resource pack. However, one screen, the one that displays "Loading terrain", was changed to having the blurred background despite previously having the dirt background. It's completely inconsistent with the dirt backgrounds of everything else while loading a world. 

Edit: The 'Saving world' screen also shows the blurred background instead of the dirt background.