After test with full Samsung 30Q at 50% mode, LED burned.
Test was 45 minutes in room. Battery voltage at end test - 3,72V.
LED burned when after the test, I changed to group mode 3 (always 100% at start) and turn on the light. LED flashed once and stopped working.
At the moment Driver gives 3,9V to LED, but no light.
Here is picture of pill.
I would like to know what is the reason of this fail.
Not enough paste on pill, bad temperature control, low bin led, or something else.
SST40?
Yes. Stock one from Simon.
Try putting gentle but firm downward pressure on the led dome while connected to an appropriate power source to see if you get a temporary flash of light…
I’ve had many SST40s fail in a weird way which is when the emitter chip actually de-solders itself from the substrate. Often you can get them to work by pressing down on them. Obviously not a solution, but may point to a cause.
The test sounds a bit extreme. What is 50% output around 800 lumens? For such a long time in smaller light you might have over heated the LED.
Can not agree with you.
This light is advertised with temperature control.
I have had bad lack. This light was maid in Friday around 5 o’clock.
That is why there is no termopaste on hole pill. 
Output will be reduced to around 700lm due to stepdown , around 2.x Amps, compared against the test with cooling I think that’s a lower limit to the step down at least for high mode (output doesn’t increase on half depleted battery despite lower heat generation and temp was a bit over 55C), that’s about 6*7135 output.
Nevertheless I don’t believe it should self destroy in normal conditions.
In the photo there is thermal paste. Did you add it? That amount is more than enough. A standard s2+ with the 3amp driver will overheat within minutes in some conditions on high mode. Also some times temp regulation needs to be set. It may be out of calibration from the factory.
Overheating may have melted the LED weld to the MCPCB.
It happened to me a few years ago on an XP-G at 1.5A on a P60 (501B).
The led did not come off but it did not turn on. Solved with a reflow.
What good is this picture? Was the pcb screwed down? If so it isn’t a problem with the paste.
What does LED look like? Visible damage? You say burned but I suspect you don’t literally mean that and something lost in translation.
Could be a bad reflow could be LED had some dirt on it. Could be the LED is fine and driver is problem. You don’t give us enough information.
Driver is fine.
No dirt on Led, no bad reflow.
Also not visable damage on LED.
Whatever the exact problem is, result is:
This S21A can not work properly on 50% in room.
Did you try it while pressing on the dome?
Without more pictures, testing, or details, nobody is going to be able to help
The S21A in the review worked at stepped down high mode until low battery without issues, at least twice, so yours is clearly defective, it isn’t your fault.
I ordered new one. This will stay as spare part.
Or one day I will order led and driver and will repare it.