When renaming banners (using anvil) with the same name, they stack together. If you place one in the environment and destroy it, it will not stack with the others (but has the same name, color etc)

Step to reproduce:

1 - Create 2 or more banners with the same color

2 - Rename them with the same name using an anvil

3 - Check in the inventory that they are all stack

4 - Place one of them, you already have x - 1 banners stacked in your inventory

5 - Destroy the banner and collect it

6 - Check in your inventory, you have now x - 1 banners stacked and an other banner, with the same color and name, not stack