Stealing @Möoz answer
It's a fourth-wall reference for the audience.
Given that we only ever see the 'glowing eyes' effect when the in-universe characters have not acknowledged it, it tells us that this must be a feature that is not apparent to them.
Additionally, according to Paul Sammon:
Ridley Scott maintains that this effect "was strictly a stylistic device, one more bit of detailing, if you like. If the replicant's eyes really did glow like that within the context of the story, then why would you need a VoightKampff machine to sniff them out?"
-Sammon, Paul M. (2000). "VIII: The Crew". Future Noir: THE MAKING OF Blade Runner.
And of the importance of eyes within the Blade Runner movie is emphasised by Mary Jenkins when she says:
[Ridley] Scott said of the replicants' sometimes glowing eyes: "that kickback you saw from the replicants' retinas was a bit of a design flaw. I was also trying to say that the eye is really the most important organ in the human body. It's like a two-way mirror; the eye doesn't only see a lot, the eye gives away a lot. A glowing human retina seemed one way of stating that".
-Jenkins, Mary. (1997) The Dystopian World of Blade Runner: An Ecofeminist Perspective, Cite note 6