FastLED fill_rainbow - Unexpected red pixel
I just copied your exact code and ran it with 600 of my LEDs. I have WS2812B as well, and using the same data pin coincidentally.

I haven't experienced your red LED issue. I then ran it again on my LEDs after adding 

 hue++;


at line 33. 

Only problem I can see is that the last 30 LEDs at the end of the rainbow kind of flicker. Groups of 10 LEDs spontaneously flicker at once, then another 10, and so on. This could be a problem with my LEDs though because they were the cheapest one's I could find on ebay 2 years ago. The code seems to work fine for me though!