By default when a banner has a pattern on it it has the tag "BlockEntityTag: {id: "minecraft:banner"}", but when placed down and then picked back up it loses that tag. It doesn't affect the appearance of the banner or anything significant, all it does is make it so the banner can no longer be stacked with banners that haven't been placed yet, even if they are identical otherwise.

I'm fairly certain this is related to MC-251509