It appears we have a voltage difference that is fully explained by the two voltage sources opposing. If we insert an open switch before or after the bulb we have 12v difference across the bulb. If we close the switch we still have the 12v difference across the bulb. Nothing changed because of the bulb. How can we call "no change" a drop?You diagram shows a 24V battery being opposed by a 12V battery so there is 12V left which appears across the bulb.
Why do you think there is none?