The bug
The eyes of certain mobs don't glow red after they've taken damage. This is noticable in a dark enough area where their eyes glow in the dark.
This affects:
Endermen
Phantoms
The Ender Dragon
Attachments
Comments 8
From what I've seen in the code and in the default shaders, this is due to the fact that entities who use the specific eye renderer don't integrate the overlay texture coordinates (which determines the color of overlays like damage tint or creeper flashing) in their rendering pipeline. As such, it doesn't render it.
Eyes are infamous to have wreaked havoc on the damage tint behavior, just look at spiders and endermen before 1.8, and the ender dragon before 1.15.
Can confirm