Sounds like a high resistance connection somewhere in the supply or return circuit. That would show full voltage under no load whilst measuring w/o the bulb but having too much voltage drop when the 2+ amps required by the bulb try to flow. The current
drawn by the LED is much lower, so much less voltage drop, plus some LED lamps will run well down to 10 V or so, so it appears to perform normally.
Test: put the 25 watt bulb in and measure the voltage at the pins. If it is a voltage drop issue it will be easily seen.