iPhone Screen Saver Issues

As of the 6th of September 2016, my iPhone has been constantly switching itself off despite background videos running via Youtube and Snapchat. I didn't notice at first when I experienced this as my brightness was already at the lowest but after about 45 seconds of watching the video the screen will darken and then after a minute it will switch off. I know I could change auto-lock to the 'Never' setting but I use the auto-lock feature a lot as I end up leaving my phone unattended and switched on and don't want to have to keep changing the settings every time I watch a video and don't want to keep having to tap the screen whilst watching videos. Any help?

Matt