I just loaded up this pre-release and I loaded in to an creative world (from an older version).
I looked at my creative inventory under saved Items and I noticed that the banners where visualy wrong,
The shield next to that had the same banner pattern applied.
I picket up that shield and tried to recreate that banner in the loom.
When I tried to apply the skull charge banner pattern in white the texture got messed up (see screenshot)
(shield with banner out of creative inventory, banner in input slot, banner in output slot)
I see that the skull charge banner pattern overrides the whole banner with black background and the the skull with bones.
I saw the too heavy overides from other patterns too
edit1: added screenshot creative inventory and in world 1.20.6 (buged too)
edit2: 1.19.4 is affected too
Its something with the texture distribution when the game is launched
I cannot reproduce. Do you have any resource packs installed? It seems like banner patterns for you are opaque with a black background rather than transparent, as shown by the skull charge button in the loom.