It's a graphics glitch, some people noticed it immediately as soon as they got out of the prologue. Apparently when it works correctly it's immediately obvious with focus. For me I can barely make out the flame, and only could light by blindly aiming and wasting several arrows

I can't believe how hard this one is. How in the world did any one find it?

The candle light is visible here, but the candle itself isn't :/

Target this spot and the fire arrow will lock on.