As the title states, getting a type of banner out of the Creative menu is fine then once you place and break it causes the banner to have NBT data so it doesn't stack. I believe it would be the same if you crafted a few, placed one and broke it.
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Confirmed for 1.8-pre1.
edit: This also applies to banners from the creative inventory: They do not stack when placed and broken.
edit2: I've attached 2 screenshots showing the difference between a crafted banner and a placed and dropped banner: Crafted banners/banners from the creative menu do not include a tag to denote their base color.
Only half fixed in 1.8.1-pre1: Crafted banners are still affected, but creative mode banners are fixed.
Can confirm, had the same issue as well