Hmm. Is he perhaps the kind who remembers people but has trouble with names? Not sure we've seen enough to tell.
Developing on this, perhaps he remembers people not through names, but through actions. Lucian remembered her as "The girl who made the shield really well that one time" rather than as "Sierra."
More interesting, however, is her developing OBSESSION with him. Why is she so obsessed about whether he remembers her or not? Crush/Love Interest? It'll take a bit more time to tell.
It might not be so much obsession with whether he remembers her name, but surprise that he really did remember her. Looking back at the last panel of page 48:
"And I'm sorry... What was your name again?"
"Oh... Sierra Wodrose, Level One Mage-- I mean Level
Two, Sir. You were my evaluator two months ago during my exam patrol... don't you remember?"
"Ah, right, I remember now."
"sigh"
At the time I read that as Sierra thinking to herself, "Yeah, right, sure you remember," and figuring that Lucian both didn't remember her and didn't think much of her. I have the suspicion that either Sierra has some significant self image problems or she just simply doesn't have any real experience other than training and doesn't yet have confidence in herself.