Perhaps this has been brought up before, but why don't Alva's eyes glow? They changed it for Sybille Stentor. I thought it would just be some global variable for any character marked as a vampire.

I believe she's a Cyrodillic Vampire.
Movarth, the one that turned her (I believe he did anyway), was a Cyrodillic Vampire who moved to Skyrim.

I'm not 100% sure he's a Cyrodillic Vampire, but to have the glowing eyes, you need to have the Volkihar strain of Vampirism.

Alva's diary suggests as much, and while this is certainly correct from a lore perspective, I think it's more of a coding oversight on Bethesda's part.

I've heard Movarth, the rest of the vampires in his lair, and those hiding out in that H______'s Shame (can't remember the location's name, but the place you go in the Daedra's Best Friend quest) all have the glowing eyes despite being Cyrodiilic. Not sure about that couple at Half-Moon Mill, though they probably do too.

The reason for that is because they didn't differentiate the vampires in Skyrim to be counted as separate strains or species, so they all have the same powers, abilities, weaknesses, and whatnot. So the Vampire Lord strain is identical to the Cyrodiilic strain in everything but its ability to transform.

Alva's race is set in the Creation Kit to NordRace instead of NordRaceVampire, so she doesn't get vampire eyes.

This could be an oversight, or it might be an attempt to stop players immediately clocking her as a vampire when they enter the town, so that the reveal later in the quest comes as a surprise.

Indeed. Lore-wise, they are Cyrodiilic.

The couple at Half-moon are now Volkihar. I thought I noticed that Babbette didn't have the glowing eyes but maybe i just didn't look close enough. I do wish they had kept Sybille as a cyrodiil strain