Hmmm I’d say the main reason the balloons have square highlights in the photos is due to the square light sources ie. windows in the scene.
You might be able to better reproduce this by adding a reflection map of a room with square light sources in your balloon material.
To do this get any HDRI environment map of a room (feel free to make it low res and in .jpg) add it as a texture sample and connect a"Reflection Vector WS" to the UV slot in the texture sample. Then just plug it in either Color or Emissive Material pin 
A nice HDRI source: HDRIs: Indoor • Poly Haven 