Resizing the game window while the creative mode player inventory is open will move the backpack slot icon / backpack to the 2nd slot of the hotbar. Closing and reopening the inventory will return it to normal. This only occurs with the creative mode inventory, and not survival mode one.

Minecraft 1.18.2
Forge 40.0.44

Mod List:
backpacked-2.1.5-1.18.2