Whenever a banner is placed, then saved, it will not drop itself as an item when broken up return to the area.
- Place a banner
- Break banner and see it drops as an item
- Place another banner
- Save and close world
- Re-open world
- 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.