When using the loom, the default banners are sometimes affected, I don't know the exact procedure to trigger this bug, but while I was designing some banners I noticed the default banners in the creative menu had changed completely. Notice that the bugged banners look like the banners I was designing.

As you can see if I take one of these bugged banners the object is still a red banner, and it doesn't have any nbt data.

If the banner is placed down, the bug continues to affect it.

If I take the data of the block with F3+I, it copies the following:

/setblock 139 113 192 minecraft:red_banner[rotation=8]{}

Trying to craft the banner doesn't revert the bug

This bug is persistent changing worlds, but disappears when restarting minecraft. This bug also gets carried to other worlds that are opened. This bug doesn't affect other banners that you already had before the bug triggers.