Originally Posted by Waynead

I don't know if it's a similar problem, but I do know that in some vehicles in you connect a trailer that has LED lights, the trailer lights won't work properly. The solution is to wire an incandescent bulb into the circuit. Then they work. Apparently the circuit needs to see the resistance of an incandescent build to work properly.

So maybe your circuit also need to sense that resistance, hence it works when you leave one incandescent in.