TBH, I don't know if this is a bug, or if it's intended? 

But realistically it shouldn't be this way.

light from off of source blocks fade into existance, and then when you break the source block, it fades away. Now I get that it looks cool yada yada yada, but light moves at almost 300,000 Kilometers per hour. Thats virtually instant, especually to our human eyes. So for especually ray tracing which is trying to be more realistic, that is not.

(Besides, thats the cause for one of my other bug reports of the light staying there and keeping the shadow of the block. So you are probably going to need to fix it anyway, even though "it looks cool").