From a 'real' standpoint, I'm figuring that they just didn't think it through. Just needed another minimal-effort emotional story because we can't just go to the beach and jump across platforms for the fun of it, can we? I'm sure they must have some sort of fill-in-the-blanks sheet for these things. There was a [boy] with [an adorable younger sister] who wanted to [become a hero] so he could [care for his family]...


But taking it from a lore standpoint, even though it initially confused me as well, I think it does make sense that he still uses the name Tia even if he's not technically "a tia". Simply enough, it would be his name from birth, unless his mother has a reason to borrow the naming pattern of some other race instead of her own. Even if, hypothetically, she's moved away from the tribe and living on her own, she's still passed the "R" tribe-name to her children as well.

More broadly, I've wondered about this before - although the Sunseeker naming pattern is very tied to their traditional tribe structure, we see very little of the actual tribes. We can take a guess based on where the tribe-name seems common, but it's not clear whether they still exist as a solid tribe or if they've fully integrated into the other societies.