If a Glowing mob is in a section (16x16x16 chunk/section boundary area) with opaque walls, it can become invisible despite the fact that it ought to glow through the walls.

Here is a repro:

Make a basic superflat world.

teleport near x=128 z=128

run

/fill 128 16 128 143 31 143 stone hollow
/summon skeleton 135 20 135 {NoAI:1,Glowing:1,Invulnerable:1}

Notice that there is a glowing skeleton in the stone box you made. You can fly around and see him from all sides. This behavior is correct. Now do

/fill 127 15 127 144 32 144 stone outline

This makes the walls one thicker, so they go out into the adjoining chunk/section in all six directions. Now fly around the box looking at the skeleton again.

Expected: you can still the glowing skeleton

Actual: you can only see the glowing skeleton if at least two of your X/Y/Z coordinates are in the same chunk/section/chunk as the skeleton (e.g. you are directly above, directly to the side, or directly below). If you view the area from any angle, the skeleton immediately disappears as you cross a chunk boundary.

Speculative cause: a rendering algorithm is culling entire sections as 'invisible due to walls' without taking into consideration whether there may be Glowing mobs in the section. (This is supported somewhat by the fact that breaking a single block in the middle of the face of one of the walls you can see will 'fix' the problem.)

Implications if not fixed: This bug has implications for PvP (where players hit by spectral arrows can 'cheat' by hiding in certain areas), puzzle maps that rely on 'Glowing' mobs for players to find hidden treasure underground or navigate through hedge mazes, or probably various other scenarios.