I put Shulkers with colored glowing effects and invisibility at the positions of ores to show their locations through walls. Then I noticed strange and inconsistent behavior in the rendering.
Sometimes the outlines were visible as expected.
Sometimes they were not visible at all.
Sometimes a few were visible and others not. When moving over chunk borders (horizontal and vertical) the image may or may not have changed.
Sometimes it changed by just moving the head (only touched mouse).
All outlines where visible when the players head was in a wall.

Steps to reproduce:
1. Spawn a Mob(does not need to be a Shulker, tested it also with Sheep) underground.
2. Apply glowing effect for example via Spectral Arrow or command.
3. Move around until the outline vanishes. For me it only worked in the red marked Chunks in pic16. (when you move to far the Game wont render the mob)
4. Repeat at different location.
It could be that this Bug doesn't apply for you because of reasons like different hardware(CPU) or software(OS, JRE)

This Bug could be related to MC-129 (pic12), MC-97094, MC-63020
But is not a duplicate of MC-101595 as you can see in pic15

Most pictures were made on a modded server using a plugin to make work more easy for me, but I included pic 13 and 14 to show that this also works (or rather doesn't work) in vanilla Singleplayer.

Pictures:
Moving over chunk borders: 1, 2, 3; 8, 9; 10, 11
Nothing visible vs. all visible when head in wall: 4, 5
Some visible vs. all visible when head in wall: 6, 7
Possible relation: 12
Vanilla proof: 13, 14, 15