If my assumption about the issue is correct, the lens itself will go to infinity as seen and felt on the lens barrel itself, but the image on the focusing screen is not fully in focus at infinity (?). Yes?
I am no expert, but had an ST in which the entire mirror box had shifted out of alignment; symptoms were similar, negatives were never really in perfect focus. It’s fixable. I have had pretty much every Contax body except the Aria, so I don’t know if there is some inherent problem with Arias, though as many people as there are that love them, I would tend to doubt it.