The light levels are correct to begin with, but after moving outside of the chunk render distance and coming back, the glitch (see attached) occurs, and persists until a block is updated where the beacon's light is/(should be) 3 or higher.

This occurs in multiplayer, regardless of the direction in which the chunks were unloaded or reloaded. In SINGLEPLAYER, the same glitch occurs, but only to some chunks, sometimes – in multiplayer, the glitch occurs whenever tested.

The beacon is at block 0 64 0, and thus in chunk 0 4 0. The iron blocks and the stone brick blocks at y=63 are rendered as dim to the north and west of the beacon (in chunks -1 3 0, -1 3 -1, and 0 3 -1). Standing on them and checking the light level according to F3 menu (at my feet, which would be in chunks -1 4 0, -1 4 -1, and 0 4 -1) gives a block light level of "0". The grass around the beacon (y=62; also in chunks -1 3 0, etc.), however, display the correct light level and are rendered correctly.

(In attachment, time is set to 22500; this makes the glitch more visible, but was discovered when time was set permanently to 23500. Setting the time is not enough to correct the lighting glitch. North is up, looking down from above.)