FastLED library with WS2812B Ledstrip. different behavior uno mega

Hi,

I have an arduino uno and when i connect it with pin 6 to my led strip everything works fine.
when i upload the same code to my mega 2560 i get everthing what should be black in green. and red is completly gone.
when i connect the ledstrip to pin 3 it works like normal.
But why is this.

because i'm busy designing a pcb with a atmega1284 but because of the layout i want to use 6 or PC3.

Johan