Ominous Banners earned before 1.16 show color patterns in text when hovering over them in the inventory, while post 1.16 banners do not, causing them to not stack with 1.16 Ominous Banners. This bug can be replicated by getting an Ominous Banner in 1.15 and then updating the world to 1.16, and then getting an Ominous Banner then and checking to see if they stack.