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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Can confirm, had the same issue as well
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.
Confirmed for 1.8.
Only half fixed in 1.8.1-pre1: Crafted banners are still affected, but creative mode banners are fixed.
Affects 1.8.7.
Affects 15w49a, and also affects banners crafted in Survival.
Affects 15w51b.
Affects 16w02a.
Affects 16w04a.
Affects 16w05b.
Affects 16w06a.
Affects 16w07a.
Affects 16w07b.
Affects 1.9-pre1.
Affects 1.9-pre3.
Affects 1.9-pre4.
Affects 1.9.2.
Fixed in 16w36a.
This bug is happening again in 1.14.3
Can confirm this is happening in 1.16.3, in survival.
Crafted banners stack, but when you place and break them those form a new stack, and don't stack with unplaced banners. 1.16.4