The bug
Retracting piston arm incorrect lighting is back in 1.11.2 (MC-11023).
This is not an issue with no smooth lighting, but it is easier to observe when using the setting "smooth lighting off."
How to reproduce
Place a sideways facing piston on the ground
Place a solid block next to the piston
Place a lever on that piston
Activate the lever and observe the side of the piston head
Affects 20w29a, can I request ownership?