The bug

Whenever a banner is placed, then saved, it will not drop itself as an item when broken up return to the area. 

How to reproduce

1. Place a banner
2. Break banner and see it drops as an item
3. Place another banner
4. Save and close world
5. Re-open world
6. Break previously placed banner and see it does not drop as item

This happens with the banners in end cities too - they will not drop as items if you leave the area/close the world and come back to them.