Summary:
If you have nothing in your offhand and craft an item that can be placed in the offhand (shield with banner, fireworks), instead of going into your inventory or hotbar, it goes straight to the offhand. (even if you have same items in your inventory already)
Updated by EVGENSYPERPRO:
Similarly, items move to the offhand when trying to move them with "shift+click" from the crafting grid.

Steps to reproduce:

1. Clear the offhand slot.
2. Craft fireworks or a shield with a banner in your inventory.
3. Clear the offhand slot.
4. Place an offhand item in the crafting grid in your inventory and try to move it to inventory (shift+click).

Observed results:
The item is moved to the offhand slot instead of your inventory.

Expected results:
The item will be moved to your inventory.

Video:
https://youtu.be/uemJHqJzwW4