A day-after purchase, flashlight stopped working. I just turned it on, tested and turned off. And the next day it didnt start at all. I tried to turn it from outdoor tools, preinstalled flashlight tool and from icon on top panel (near wifi, airplane mode and etc) - no effect. The button/icon draws as pressed, but LED doesn't lights. Reboot didnt help too.
I've read in this forum, that it's a hardware problem/poor contact, so i let the problem loose.
But yesterday, while serfing internet, flashlight suddenly started to light by itself. So i wondered at it, then manually turned it off via flashlight tool, and then it didnt turn on again!) Shaking, knocking the phone didnt do anything too. I still suppose, that it is some sort of software bug?

It's an hardware fault at 99%.
Before contacting your seller for a replacement, you can try to update your device to the latest firmware version and to perform a factory reset after that.