A desync occurs when you press F (or whatever button switches hand) as soon as you place an item in a hotbar slot in the creative mode inventory.

Steps to reproduce:

1. Go into creative mode
2. Pick up a stack of items from the creative inventory into your cursor
3. Drop that stack into an item slot, and immediately press F
4. Exit the inventory and navigate to that hotbar slot
5. Even if it appears empty, you can press Q and drop that item, and the item remains in the offhand anyway

Can not reproduce in survival, but it might be possible