Thanks for the explanation. I tested two different Samsung INR21700-40T which work perfectly fine in other flashlights. That’s why I think the spring can cause this.
Funny, I PM’d Simon and told him about the rattling bezel in my M26 and M21 and he said he never heard of the problem. I managed to tighten the bezel enough but I did it so hard I though I was gonna break it.
I did that too, it hepled. But yeah, it is annoying. And it is rattling reflector, not bezel. You can get thicker o-ring and solve this problem (o-ring under the bezel) 
Have you tried switching to 4 brightness mode? Click the side switch 6 times to do that, maybe one of your mode groups is broken.
Unfortunately same problem 
If you put on a thicker rubber O-ring, it will only get worse. The reflector is not pressed by the O-ring, but directly by the glass. In order for the glass to come into contact with the reflector, the glass must first strongly compress the O-ring, which is pressed against the body of the lantern around the reflector.
Now in July 2021, has the ramping mode been smoothed to get an even increase & decrease in ramping? In both the M21-C-U and the M21D?
Controlling the led current by PWM, where the duty cycle control is being applied as linear, would cause this perceived problem. This does apply also in changing the volume of sound, it is a logarithmic power increase for a smooth perception
volume increase; the db scale. that is why older (I date myself) analog volume potentiometers needed a logarithmic taper.
2nd question, are the M21C-U and M21D identical except for the addition of a usb charging port?
Edited in, 3rd question. Since this is an electronic off mode, instead of electrically removing all battery voltage, what is the leakage current in the off mode, typical or maximum??
Thanks, Stan
1 & 2: I will get my M21D soon, so I will be able to find out I will let you know! And you are right about this linear problem, Simon told me it will be fixed, so we will find out.
3: You can unscrew the tailcap to physically disconnect battery. In other case I don’t know, but it should be typical.
Thanks Mocarny, I am very close to purchase. With cmos circuits, even although leakage is very low, the will be a leakage current. It could be several microamps or just a fraction. Not knowing just how much it discharges the battery, OK I just calculated it. If the leakage is just 10 microamps, the the battery will discharge 0.08 ah in one year. Guess that is very acceptable. Battery self discharge would be more.
If the M21C-U is the M21D (has usb charge) I think I would want the M21C-U just for a sealed flashlight. Are you getting it with XHP70.2? Color temp?
I’m getting XHP70.2 4000K - I have a 4X18A XHP70.2 5000K so I will compare the CCT of those two.