Sharp image library rotates image when resizing?

When using the sharp image resize library https://github.com/lovell/sharp for node.js, the image is being rotated.

I have no code thats says .rotate(), so why is it being rotated and how can I stop it from rotating?

I'm using the serverless-image-resizing example provided by AWS: https://github.com/awslabs/serverless-image-resizing that uses lambda to resize images on the fly if the thumbnail does not exist

S3.getObject({Bucket: BUCKET, Key: originalKey}).promise()
.then(data => Sharp(data.Body)
      .resize(width, height)
      .toFormat('png')
      .toBuffer()
    )
.then(buffer => S3.putObject({
        Body: buffer,
        Bucket: BUCKET,
        ContentType: 'image/png',
        Key: key,
      }).promise()
    )
.then(() => callback(null, {
        statusCode: '301',
        headers: {'location': `${URL}/${key}`},
        body: '',
      })
    )
.catch(err => callback(err))

Original large image:

Resized image: note it has been rotated as well:

The problem actually turned out to be this: when you resize an image, the exif data is lost. The exif data includes the correct orientation of the image, ie which way is up.

Fortunately sharp does have a feature that does retain the exif data, .withMetadata(). So the code above needs to be changed to read:

S3.getObject({Bucket: BUCKET, Key: originalKey}).promise()
.then(data => Sharp(data.Body)
      .resize(width, height)
      .withMetadata() // add this line here
      .toBuffer()
    )

(Note that you also need to remove the .toFormat('png') call because png does not have the same support for exif that jpeg does)

And now it works properly, and the resized image is the correct way up.