Illager banners have a black border

Read summary. Prior to 18w47a, they didn't have the black border. (Called "Black Bordure" in the tooltip.)

The reason I think this is unintended is due to the fact that Mojang never announced these changes. And don't argue that "Mojang never announces minor changes such as these," as in each snapshot page they clearly have a list of fixed bugs, some of which aren't very major.