I just tried out 18w22c today and seem to be having major issues with block lighting.

I first noticed it on my server (was an 18w14b world).  I added a roof to the top of a wastewater screening/liftpump building and the lighting on the floor remained at 15, even with the room sealed off( no half slabs).  This persisted until I lowered the level of the floor down around 10 blocks to reach the sewer depth.  Very strangely, digging a block or two out sideways from the below grade room, and light could be found again.  (There is an aeration basin at surface level next to the building though since water seems to be causing some anomalies)

I also noticed the city water line tunnels, which is normally as dark as it can get, had abrupt patches of complete light.

Next is my 18w16a map:  Almost no shadows in the forests composed of big trees, and I mean big as in custom built tree models painted there with world painter.  I sealed myself inside a tree, and same story, It is completely lit.

Edit:  The blocks themselves are giving off level 15 light.  This is almost like using the applyblocklighting filter in mcedit, except it's everywhere and without choice.