While in inventory menu, items equipped to character cannot be moved into inventory by hitting X button even when there is available space to store items. Also the controls guide located on the bottom ribbon while hovering over shoulder slots is also missing the option “x button to inventory”.
For other slots (eg gloves, hat, mask, armband, boots) the controls guide on bottom ribbon while hovering over these slots does have the option “x button to inventory”, however these items also do not move when using x button even when there is available space in other inventory slots.