Summary:
Creative items are ignored in Grindstone. Because of this, it is also impossible to get a shield without a banner (when combined with a shield with a banner) and untrimed armor (when combined with trimed armor).

Steps to reproduce:

1. Take an 2 unenchanted item from your creative inventory (armor, shield (anything that can be placed in Grindstone)).
2. Place the Grindstone.
3. Place items in Grindstone.

Steps to reproduce (2):

1. Create a shield with a banner.
2. Take the shield from the creative inventory.
3. Place the Grindstone.
4. Place shields in Grindstone.
5. Try swapping shields.

Observed results:
1 The output slot is empty.
2 Impossible to get a shield without a banner (there will always be a shield with a banner in the output slot).

Expected results:
1 The combined item will appear.
2 Will be possible to get a shield without a banner.

Screenshots: