Demonstration (option toggled off/on): https://imgur.com/a/8TnhwHz

Issue doesn't occur when one of the slabs is broken: https://imgur.com/a/84xZHui

FYI - fix for this will wait for v0.7 or later because it will be handled when smooth lighting is redone to handle colored lighting.

Suspended for now - likely to replace entire lighting method soonish