What's the story of the BME680?

Other Hardware Sensors
2

Yes.

Here is a basic IAQ formula. Notice how the IAQ is weighted by humidity. That's why there is a track with humidity.

float fCalulate_IAQ_Index( int gasResistance, float Humidity)
{
  float hum_baseline = 40.0f;
  float hum_weighting = 0.25f;
  float gas_offset = 0.0f;
  float hum_offset = 0.0f;
  float hum_score = 0.0f;
  float gas_score = 0.0f;
  gas_offset = oGasResistanceBaseLine - float( gasResistance );
  hum_offset = float( Humidity ) - hum_baseline;
  // calculate hum_score as distance from hum_baseline
  if ( hum_offset > 0.0f )
  {
    hum_score = 100.0f - hum_baseline - hum_offset;
    hum_score /= ( 100.0f - hum_baseline );
    hum_score *= ( hum_weighting * 100.0f );
  } else {
    hum_score = hum_baseline + hum_offset;
    hum_score /= hum_baseline;
    hum_score *= ( 100.0f - (hum_weighting * 100.0f) );
  }
  //calculate gas score as distance from baseline
  if ( gas_offset > 0.0f )
  {
    gas_score = float( gasResistance ) / oGasResistanceBaseLine;
    gas_score *= ( 100.0f - (hum_weighting * 100.0f ) );
  } else {
    gas_score = 100.0f - ( hum_weighting * 100.0f );
  }
  return ( hum_score + gas_score );
} //void fCalulate_IAQ_Index( int gasResistance, float Humidity):

I do not use the BME680 to do IAQ. The BME680, to create the Index, figures out the Index with the the first 50 or so readings. Thus IAQ is influenced by your start up Air Quality. Do not forget the meaning of the word Index.

I created my Index gas resistance value by taking 100's of readings during the wee hours of a spring morning.

Mashing stuff onto the BME680 will mess with its ability to do IAQ readings. The IAQ readings are done by hot wire.

Here is the Indexed value I use for IAQ, const float oGasResistanceBaseLine = 149598.0f;