Unexpected slashes in Azure JSON

Hi, I’m using a Photon with a BME280 sensor to send weather readings to an Azure IoT Hub using the particle integration, but I’m struggling to figure out this ‘slash’ issue.

I have this code:

#include <Wire.h>
#include <SPI.h>
#include <Adafruit_Sensor.h>
#include <Adafruit_BME280.h>

#define BME_SCK D4
#define BME_MISO D3
#define BME_MOSI D2
#define BME_CS D5

#define SEALEVELPRESSURE_HPA (1013.25)

Adafruit_BME280 bme(BME_CS, BME_MOSI, BME_MISO, BME_SCK); 

unsigned long delayTime;
float temperature = 0;
float pressure = 0;
float humidity = 0;
char buf[256];

void setup()
    Serial.begin(9600); //initiate serial monitor and set data transfer rate
    bme.begin(); //initiate the sensor
    delayTime = 10000;  //set delay to 10 seconds

void loop()
    //take sensor readings
    temperature = bme.readTemperature();
    pressure = bme.readPressure() / 100.0F;
    humidity = bme.readHumidity();
    //clear JSON buffer
    memset(buf, 0, sizeof(buf));
    JSONBufferWriter writer(buf, sizeof(buf) - 1);    
    //write readings as JSON
        writer.name("Temperature C").value(temperature);
        writer.name("Pressure hPa").value(pressure);
        writer.name("Humidity %").value(humidity);
    //print JSON buffer to serial monitor
    //publish readings to Azure
    Particle.publish("SendToAzure", buf, PRIVATE);
    //wait some time

which is successfully creating valid JSON:


but when I check the events arriving in the IoT Hub the key string has gained a slash before each double quote:

I’m a bit new to this so not entirely sure how to resolve my code to generate valid JSON in Azure (i.e. without the slashes). Any pointers will be much appreciated!