I am having a weird bug with Project Red and Thaumcraft. I planted a silver wood tree and when it grew and spawned a pure node. Then the Project Red lamps ~4 blocks below it stopped preventing mobs from spawning. See picks below. On the final screenshot I show the mini-map and the silver wood tree is dead center in the trouble spot.

Haven't played with thaumcraft as of late, but I'm pretty sure that a pure node starts to change the area to a magical biome. I think its because of that. Either way, there isn't anything a lamp does that would cause this. Try using glowstone blocks instead of lamps and see if this still happens.

I am pretty sure now this is a thamcraft bug. But i wanted to check with you just in case it wasn't.