Since the day banners have been introduced this BUG persists.

Resizing the default MC banner_base.png from 64px up to 128px works fine in game.
As soon the texture is resized to 256px it stops showing colours for banners.

When I scale down the texture back to 128px it works again.

It is not about the colour depth... read what I wrote... I am doing this with the default MC texture... you can reproduce this bs yourself. It is not a resourcepack issue.