When a banner is pick-blocked, it does not stack with other banners with the same design created before 1.16

When a banner with a design is placed down, then the pick block key is used on it, the game creates a new banner in the player's inventory (while in creative) and this new banner (with the same design as the original banner) does not stack with the original banner.

 

EDIT: See comments for information on how to reproduce.