Flashlight turns itself off while a recording video on iPhone 5s
The iPhone flashlight uses the iPhone flash to illuminate your path.
  This uses an LED to provide the light, which lasts longer than a bulb
  you would place in your house or one that you would find in a cheap
  flashlight.
  
  According to Bulbs.com, “Many LEDs have a rated life of up to 50,000
  hours.” To put that in perspective, a typical LED can last almost six
  years of constant use. It is unlikely that you will use the iPhone
  flashlight for six year’s straight, so there is little chance of the
  iPhone flashlight burning out or damaging your iPhone camera flash.


http://www.gottabemobile.com/2015/02/16/iphone-flashlight-tips/

The best to do is to contact the application's Developers to report the issue ;)