Confirmed for 14w05a
Yes I did to test if the lc value bug still exists. But flying further revealed more mushrooms generating in chunks where the lc value has not been raised.
Edit: Th lc value from the screenshot is not from the block I placed but from the mushrooms that generated.
I can confirm that this issue still exists in snapshot 14w04b.
I tested this by getting a beacon to work in the nether without breaking bedrock, going on top of the ceiling, placing and breaking a block in the beacon chunk and reloading the world to reset the lc value.
When i got back to the beacon the lc value was reset but the beacon did not work any longer.
Confirmed for 14w11b