I did some testing on my new H600Fc Mk IV using a Sanyo NCR18650GA, and let the headlamp rest between tests to cool down.
I did not have anything like the flickering seen in the YouTube clip or failing to enter H1, however the tail cap is (possible) defect as I also tested using tail cap from SC64c. Usually a lockout on my Zebralights will occur almost immediate when turning the tail cap but on the H600Fc I have to turn almost a quarter turn before lockout. Looking inside the tail cap I see that some "goo" (glue?) that I tried to scrape a bit off.
1) For battery voltage to about 3.7v on H1 I could see visible step downs after about 42 seconds for the original tail cap. No testing with SC64c tail cap.
2) From about 3.58V the H1 would jump to M after about 25 seconds using the original tail cap, but not when using a tail cap from my SC64c (step down after about 55 sec but not to M).
3) At 3.54V the H1 would jump to M after 16 seconds, but using the SC64c tail cap would last 55 seconds for visible step down but not jump to M.
4) At 3.45V H1 goes to M1 after 5 seconds. For the SC64C tail cap the was step down to M after 33 seconds.
5) At 3.39V H1 goes to M1 after 3 seconds. For SC64 step down to M1 after 13 secs. Using SC63w tail cap step down to M1 after 3 secs.
So, I'm very pleased with the tint of the H600Fc and would just like to have a new tail cap and not return the headlamp.
Edit: Added a test using a SC63w tail cap that looks identical to H600Fc tail cap. It performs the same, so perhaps wrong tail cap issued with the H600Fc?
Edit: Since the spring in the SC64c tail cap is shorter than the original tail cap a hard shake can make the light turn off.