Blocks are rendered brighter on Z axis (north-south) than X axis (east-west)

See attached screenshots; for some reason the X sides of a block are rendered dimmer than the Z sides of a block. Not sure if it is intended, but I think they should look the same.

Affected blocks include regular blocks, signs, banners, chests, walls, etc but not blocks with crossed textures (like crops, saplings and cobwebs).