All light levels are displayed as 0 when on soul sand

While standing on soul sand, the light level values of both the client and server light are displayed as 0, despite there being light sources in the area. The type of light source does not seem to matter. Reloading the game does not seem to work, and whether the player is wearing enchanted armour or not also has no effect (thought at first it might be due to Soul Speed enchant but doesn't seem to be the case. Tested with other blocks and it seems that only soul sand has this bug.