That is not very surprising! Beside that Darius fell in love with a house girl and wanted to get her out of this "den of sin", he certainly was not big on gaining friends or making a good impression. Hana disliked him even more than Ian(!) and even Ian himself was rather lukewarm when he told Edwin about Darius. Only after pushing Ian towards the reform path our bro starts to dig around himself too.
Technically Ian will still dig into Darius' papertrail even if the MC doesn't try to reform him (presumably he got a similar, if darker, push from Chuck's phone call in that case).
That said, I agree the relative lack of concern over Darius rings fairly true. From all accounts he was a very abrasive personality who tended to act on impulse. Given that he wasn't from the area originally, it's no surprise most people would accept it when he apparently pulled up stakes and left; they were simply happy he was gone. Ian was the only person who might have reason to look into it, but Ian is both lazy and confident the Club would never do anything... drastic. He probably looked at it as giving his friend space and assumed Darius would call him when he wanted to talk about it. By the time the lack of communication would have become suspicious Ian had moved on and no longer considered Darius's departure relevant... until the MC *forced* him to reconsider the whole event in a more sinister light.
Overall the situation makes sense given the people involved. Personally I think the biggest question in the sequence is why the MC is suddenly so obsessed with Darius just because van Doren raised the subject during his visit. Unlike Ian, the MC was new to the Club when he learned of Darius' disappearance. He didn't have time to build up a casual comfort with the criminal enterprise before being asked to consider how far it would go to protect itself; indeed, he immediately accuses Kathleen of foul play the instant he learned Darius was blackmailing her. If the MC was willing to turn a blind eye to the whole thing after that revelation I don't see why van Doren's cryptic clues would instantly make it a priority. Sure, van Doren obviously wanted the MC to connect the dots, but it's unclear why the MC trusts him enough to let that warning override the tacit decision he'd already made about working for the Club.
I presume the idea is that Abel's visit was sufficiently traumatic that, like Ian, it shook the MC out of the complacent rut he'd fallen into, but I think both the rut and the trauma were a little underplayed. On the other hand, I say that as someone who loathed the Club from the get go so it's entirely possible my own agenda is coloring my take on the MC's complacency.
Either way, it's not a serious problem and it certainly supports the game's pacing to bring Darius to the forefront now.