Check if the LED removed itself from the PCB? Happened to me once but that is the fault of my shitty reflow. It seemed to be still attached slightly but I tried to prod the LED with a tweezer and it was loose. Might worth checking.
Looking at the led front from the lens, it looks as well-placed as it was new….the driver on the other side is still tight, I held the spring on the + plus side and it is tight, no rattle or any small movement.
On further searching, it is a protected cell with circuit board at the cathode end. A fault in the bms would likely disable the FET in the discharge path; that’s why an external load test would be needed to rule that in/out. Just reading the OCV of a protected cell doesn’t prove that it is functional or “good”.
If the cell protection has activated, then the cell will not drive current thru a load.
If the cell will deliver 1 or 2 or 5 Amps during a load test, then the cell is likely OK and the problem is in the flashlight.
To load test a cell, you will need some wire and a low resistance device to draw current, e.g. a 20W 2 Ohms resistor would draw about 2 Amps, or you might connect an old car headlight (12V, 50W) or bulb across the cell as a load. Note that this will create some heat so watch out for your fingers when touching the load.
Use your voltmeter to monitor the cell voltage while it is loaded to watch it during discharge and stop the test if the cell voltage goes too low. If you meter will measure current, then you could monitor that also to get a reading for the load.
I might try the oem, protected Klarus 26650 cell with my other 26650 light and see if it will work, for as previously suspected, it might display voltage but just not enough amps to power the indicator switch lights but not the main led.
I don’t know if the “power tube” and the “signal tube” of your Klarus function the same way as my humble C8’s. You could try to touch the blob at the end of the black lead on the leadboard with a pen probe connected to the “shell” of the light… In the C8 this shunts the driver, causing the led to burn on direct drive. If (big IF) the light burns, it’s probably something in the driver or a connection. If the light does not burn, it could be anything.
(very) Remote possibility, this happened to me just once. The copper core in one of the leads going to the led is broken.