*This has nothing to do with the pick block on a colored banner will still give you a black banner bug. It is different.

To replicate:
Place a banner that has a pattern. (As in you have crafted it with other things so that it isn't one solid color anymore.)
Sneak and pick block it (this makes most blocks retain nbt data, so chests keep their items in them, signs keep their text, etc.)
You will still get a blank banner. (All one color)

If I had to guess, I would say that this is because banners have their nbt data saved differently than other blocks, because they already have different items for different banners and because they can be copied, like maps, but I don't know for sure if they are saved differently.