From time to time, light emitting blocks will fail to have their top side lit. Oddly, this glitch did not affect the beacon. I have so far failed to repeat the glitch, but the majority of blocks were placed during the day, and any subsequent blocks placed that emitted light, excluding the beacon, were also glitched. However, it only seemed to affect a certain patch of land, so it might have to do with the chunks they were in.