A lone bulb under the playfield is a load for the solenoid expander: leave it in

The bulb wired under the playfield that never shows on top is there on purpose. It sits in parallel with the solenoid expander's input so the lamp driver SCR feeding it draws enough current to latch. If it is removed or burns out, the expander can stop switching reliably and several solenoids may stop working, so check it when expander solenoids act up.