When copying banners onto blank banners, the game allows you to place other items in the crafting grid, and these items will be used when the banner is copied in addition to the blank banner.
The crafting will use as many of the item provided up to the number of banners crafted, and will use one of the item for every banner crafted. It can also do multiple stacks of items.

The pictures provided are before and after shots demonstrating the bug before and after crafting.
Here is a video that I quickly made to help document this bug:
https://www.youtube.com/watch?v=Cvf-R0dPw-8

To Reproduce

1. Give yourself a blank white banner:

   /give @s white_banner

2. Give yourself a white banner with a pattern:

   /give @s white_banner{BlockEntityTag:{Patterns:[{Pattern:ss,Color:15}]}}

3. Open a crafting table and place both banners, plus one or more non-banner items, into the crafting slots
4. Collect the copied banner from the crafting output slot
   → Some or all of the non-banner items are consumed

More specifically, one item is consumed from each stack per banner copied.

Darth Cobby in this comment