if the brightness is below 30, mixed colors are no longer output

[SignumV6]

In the control, the brightness is regulated by a sensor.
If the brightness falls below 30 (colorCHSV.val = 30), mixed colors are no longer output.
It is e.g. with colorCHSV.hue = 80 pure green displayed.

[5chmidti]

I've ran

  fill_solid(leds, NUM_LEDS, CHSV(80, 255, 20));
  FastLED.show();

and couldn't reproduce your results (tested with FastLED.setBrightness(255);). Yes the color is quite green, but the red part of the led still lit up. If you're using this to control global brightness, don't. Use FastLED.setBrightness(brightness); instead in that case.
A lower brightness/v of a CHSV object results in lower resolutions in hue due to the nature of scale8/scaling bytes, which probably is what you mean.