What I expected to happen was...:
That the light level of the lantern would stay the same while being moved

What actually happened was...:
The light level of the lantern drops from 15 to 0 while being moved

Steps to Reproduce:

Note: please check "snapshot lantern lighting bug.png" for a picture of the redstone circuit

1. place a lantern on the ground
2. place a piston facing in the direction of the lantern next to the lantern
3. place another piston facing the lantern on the opposite with one air block in between

( View: P L A P)

(P=piston, L=lantern, A= air block)

4. place a repeater facing the first piston

5. place a repeater facing the second piston

6. connect the repeaters from step 4 and 5 to repeater clock

7. Attach a button to one side of the redstone clock

8. Press the button once and break the button quickly

9. Open the F3 menu and observe the Client light level (Block)