Try moving the sensor around very slightly on the meter, also rotate it on it’s axis (ie cable entry down, up,left and right etc) and/or angling it slightly whilst watching the LED.
Since you have now observed this odd behavior I shall also share my findings (I didn’t want anyone thinking I’m even more nuts than they already did)
I have noticed a strange situation, where I can move a sensor and position it to give anyone one of 3 outputs from the same pulsed light source,
- LED flash only, no pulse via wiring,
- Pulse via wiring only, no LED flash or
- Both and LED flash and pulsed output from wiring.
(obviously not including the 4th possibility of no led flash or pulse output if not positioned anywhere near correctly)
I had tested this further with a standard led controlled by Arduino blink.ino in free air (this rules out any possibility of 2 different light frequencies/intensities being the cause) and found that once located by moving the sensor. I can switch between the 3 results at will by moving very slightly. I assume either there are 2 actual sensors on board (which makes no sense) or the switching circuitry for the LED or pulse are independent of each other and have different characteristics (but that doesn’t directly explain the effect that positioning has, but may indirectly?), since my findings were done in free air with a gap between them, ambient light may play a part too.
I have only tested this on one sensor, the first and only one I had at that time so whether it is apparent on all sensors I couldn’t say, I know it sounds bizarre.
Bottom line is if what you are seeing is the same as I found, then you should be able to play with the positioning and find that “sweet spot” in which both the LED and pulse output work.
I had asked for a circuit diagram but @glyn.hudson has said the manufacturer will not provide one and as they are sealed units, I never got to the bottom of it.