You guys had the wrong LED type in the arduino setup:

//   NEO_GRB     Pixels are wired for GRB bitstream (most NeoPixel products)
//   NEO_RGB     Pixels are wired for RGB bitstream (v1 FLORA pixels, not v2)
//   NEO_RGBW    Pixels are wired for RGBW bitstream (NeoPixel RGBW products)

I have the 12V GRB option - one controller connected to 3 leds but it runs smooth as a cat. With RGB and RGBW it doesn't work that well and I see only 3 led chunks. ali express link 12V2811 5m |1 50 leds | White PCB

That is the setup line that works for the WS2811 RGB strip (GRB strip):

Adafruit_NeoPixel strip = Adafruit_NeoPixel(NUM_LEDS, PIN, NEO_GRB + NEO_KHZ800);

I went on this by accident and I thought I should let you know even after 2 years :-) Hope that helps