WS2812B with FastLED shows only 1 first led

Other Hardware LEDs and Multiplexing
16

I tried to minimize the code and this is the simplest block of code with my problem.
If there is 1 show(), only first led lights up
I added one more show() and it works as expected.
Even in the FastLED Blink example, if I want to blink more than 1 led, it always lights up only the first led in the first show() . The subsequence show() can lights up the other leds.
For more details, I'm using Esp8266 Wemos D1 Mini, WS2812B and FastLED library

#include <FastLED.h>

#define NUM_LEDS 24
#define DATA_PIN D6

CRGB leds[NUM_LEDS];
void setup()
{
  uint8_t i = 0;
  FastLED.addLeds<WS2812B, DATA_PIN, GRB>(leds, NUM_LEDS);
  FastLED.setBrightness(50);

  for (i = 0; i < NUM_LEDS; i++) {
    leds[i] = CRGB::Black;
  }

  for (i = 0; i < 7; i++) {
    leds[i] = CRGB::Red;
  }
  FastLED.show();   //1st show() lights up only the first led
//  FastLED.show();   //2nd show() works as expected
}

void loop() {
}