it could be. But I can get that "rubber band effect", if it is what I think it is, with my flybars too, even in Acro mode (or with no Pixhawk at all) if I run the head too slow.
Being fixated on running excessively low headspeed to get 1 minute more flight time is counter-intuitive to having a stable helicopter at the expense of a little higer fuel burn. And it's not that much. I'm kind of new to testing electric helicopters, as I like piston engine power. But flying my Trex 600 at 10.2lbs takeoff weight with 8300 6S in it I get 16 minutes if I run the head at 1, 800 or 1,900. Doesn't make one bit of measurable difference except that I get way less blade chatter in the corners at 1,900 rpm. Turn it up to 2,000 rpm and it's smoother yet at the expense of 45 seconds of flight time. And that's still far below the typical 3D headspeed of 2,300-2,400 rpm that most would run with a 600.
The ESC governor simply doesn't work right below 70% throttle setting (1,800 rpm). And I question if Tim's is working right. In the one vid I saw him fly, with an unloaded helicopter, he pushed in a fair amount of positive collective and I could hear it audible lug the motor when he did that. Mine will only do that if I use a throttle setting lower than 700 in the params. At setting of 700 or above it will hold perfect headspeed at full collective pitch, regardless of battery voltage.