The sensor circuit was designed incorrectly
Replace the sensor with the new type, and upgrade the Gemini 2 firmware to 2.14 or above. After replacing the sensor, an adjustment may be necessary to pot R27 on the Gemini 2. Place the stock (not the black mark) over the sensor and measure test point MP4 to ground. Adjust the voltage to >0.5V. If you move the black mark through the sensor, the voltage should increase. We may never see this problem on an Apollo 3 printer, since we only received a few Apollo 3 printers before this problem was fixed, but the Apollo 3 does use the same sensor.