I was messing around with the settings included in the deferred technical preview, when I noticed a hard light edge around the block that appeared with the Shadow Quality settings set at or above High (or the point where point light shadows are enabled) when the player is within 7-9 blocks of the source. Increasing the point light quality had no affect on this phenomenon, but took a large chunk of performance.

All of the resource packs I tested (Vanilla PBR and Lynx Deffered PBR) all had the same effect.

This also affects other blocks such as shroomlights, glowstone, beacons, and sea lanterns.