Summary:

The eyes of such mobs as enderman, spider and phantom are not affected by the darkness effect.
Also, the effect does not affect mobs such as magma cube and blaze, as they also emit light.

Steps to reproduce:

1. Set the night.
2. Give yourself the effect of darkness (/effect @s darkness 1000000 100 true).
3. Summon mobs with glowing parts / glowing mobs (eg spider, enderman, blaze, magma cube).
4. Move away from the mobs.
5. Consider mobs.

Observed results:

The glowing parts of mobs / glowing mobs are not affected by the darkness effect.

Expected results:

The darkness effect should affect all mobs.

Image: