Theory: The driver has a protection function that shuts the FL off when the voltage drops below ~4v. Around 3v the driver doesn't have enough voltage to operate, so no protection, and current leaks through the junctions.
That almost sounds plausible. 
I’ve played with the light some more, it’s pretty good. With 3x18650 it’s as bright as my custom DD MTG2 C8. It’s obviously bigger/wider than the C8 even in stubby mode and very heavy at the head, which is good for heatsinking. Build quality is excellent for the cost.
As some minor downsides, because this light uses so many large cells, the springs at the head and the tailcap are not strong enough to completely maintain tension on the cells. As a result, if you swing around the light even a little, the modes change because spring contact is momentarily lost. Thus this is not really a good light for anything where reliable operation is paramount. As a work around, I think 3x18350 cells will fit in stubby mode so I have cells on order and will do more testing. The obvious trade off is reduced runtime.
Also the pill does not appear to easily opened so I have just left it. I think it was mentioned here you have to remove the driver first to unscrew the reflector, but I don’t want to start prying and gouging the PCB.
It looks like there’s a screw-in retaining ring for the driver, it might not require much prying. And you’re right on the modes changing like that. On the plus side you can just shake it to change modes, no messing with the tailcap. 
I wonder if one can find stiffer springs for the tailcap/pill.
Mine has a pressed in retaining ring holding the driver. I was able to carefully pry it out with some fine angled picks.
Hmm…
I see a toroid. A big one. That means this is a boost driver, right? Then why does current draw decline linearly with voltage?
I bought mine before I knew about these websites, manafont, cnq, aliexpress etc…. Mine was a clone and didn’t do so well. Not that it’s bad or anything, still working but I gave it away. Not sure about this seller is selling real ones or not but I would spend 5 bucks to get the extension tube for 3 batteries.
I just bought 3 extension tubes for $8.19 at the Banggood sale. 
TR-3T6 on 5x18650??
Seriously though, does anyone know of a nice, spicy driver to put in this pill?
That driver is the right diameter but the ‘T’ part is about 5mm too deep to use in the 3T6 pill. I want a boost driver, something that’s going to keep a constant wattage to the emitters until the battery stack drops below 8v.
No, I wanted to see what 8xAA Duraloops would do. 
Probably too long for everyday use, but it's nice to have options.
Interesting, but then you will almost have a walking stick instead of a flashlight. 