Encounter with Glas Ghaibhleann

Glas is met thrice in Generation 9:

• Alchemist Seal: Leymore and Cai see it bashing a tower beacon for no apparent reason.
• Deliver the Ice Mines: A Milletian sees it bashing a house, then it spots the Milletian and tries to charge them, but steps on an ice mine and rears back with that loud annoying screech before vanishing.
• Scouting Mission: Five Milletians fight and kill it.

What was Yvona's lines of dialogue? Five Milletians, right? It'd probably coincide with Scouting Mission.

Well...I guess it would be scouting, but glas was still alive. she didn't say it died or anything, and Glas could be roaming around. The Milletian (Not the ones with Yvona) was the one that ended up killing it, not the 5 that accompanied Yvona, which can safely assume failed and rebirth elsewhere since Yvona never saw them again, and vice versa. And besides...Yvona+5 = 6.

Do note that I said, sudden appearance, not directly referencing anything. I'm not sure why you linked it to begin with, since she could have encountered it anywhere.

Didn't she run away during the fight though?

It could be a reference to Scouting Mission, since the number of Milletians (5), the location (Shadow Realm), and scenario (Milletians vs Glas) are the same. There's a possibility DevCAT might be reusing their ideas to build character storyline.

There's also a possibility that we don't remember seeing her, and she doesn't remember our faces (I remember she said something about not caring for others).

She did run away. There really is no other text, other than she ran while the other Milletians tried to fend it off, giving her enough time to flee.

That means she doesn't witness the possibility of Milletians being successful in killing it.