From your code, i only see one issue:
you should call bme680_init before bme680_set_sensor_setting.
gas_sensor.dev_id = BME680_I2C_ADDR_SECONDARY;
gas_sensor.intf = BME680_I2C_INTF;
gas_sensor.read = bus_read;
gas_sensor.write = bus_write;
gas_sensor.delay_ms = _sleep;
gas_sensor.amb_temp = 15;
int8_t rslt = BME680_OK;
// config sensor into forced mode
uint8_t set_required_settings;
rslt = bme680_init(&gas_sensor);
if (rslt != BME680_OK)
{
return rslt;
}
/* Set the temperature, pressure and humidity settings */
gas_sensor.tph_sett.os_hum = BME680_OS_2X;
gas_sensor.tph_sett.os_pres = BME680_OS_4X;
gas_sensor.tph_sett.os_temp = BME680_OS_8X;
gas_sensor.tph_sett.filter = BME680_FILTER_SIZE_3;
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