Like, well first of all, I went into this manga not knowing anything about it. So I honestly had the same thought initially, that his relatiomship with Mira was going in the usual romantic-feelings direction, it's not an uncommon trope in manga and I had no frame of reference for the author's intent yet lol. I was actually kinda surprised when it went for pure famillial feelings instead!

But beyond that, I think it's interesting and realistic for Julian to have a somewhat skewed understanding of love as a concept. He had never been exposed to it in any substantial capacity, and he canonically doesn't have a huge degree of romantic inclinations. But that could be somewhat related to him not having been allowed that kind of freedom with who he cares about growing up. He's had family, and he's had friends, but before that he was mostly on his own, and caring about others was a no-no, both to protect his own life and to protect his own emotions. (I think Julian couldn't stop himself from caring about the other slaves at all, no matter what happened to him, but I think it was likely he berated himself for getting attached to others in the past, only for them to get ripped away from him. Eventually he comes to understand it wasn't his fault, and that the blame should lie with the ones that hurt others, which I LOVE as a point in his character development—but as a young kid, I think he probably didn't know what to think.)

So when he finally has some form of love directed toward him—platonic or not—it WOULD be interesting for Julian to not know how to process it. He hasn't had it before, not from his own mother or anyone else, and though Mira thought of him like a child she wanted to take care of—a son, but her own history also makes that very complex, but that's a separate topic—I think it's not unreasonable to think Julian would develop a crush of some kind toward her, the first person to show him affection and kindness like real family would. Though I don't think, especially in the early manga, that he would necessarilly understand those feelings in a complex capacity. He wouldn't have enough experience to untangle and differentiate between platonic/famillial/romantic love, and he wouldn't even know it was a crush like that. Even after he meets Angvall and Titi, I don't think he ever really mentions what exactly happened on the slave ship, mostly to protect them from his own horrific past (self-sacrificing tendencies galore hoo boy).

I think in the end he comes to understand his own feelings, and regardless of "appropriateness" accepts he might've had a crush (in kind of an idolization way?) on her, but I believe it would've come from a place of loneliness rather than a place of actual romantic inclination.