I have a sous vide set up as well, and while I find that it works great for chicken and duck, I agree with you that sous vide does something weird to steak. It's something about the texture. I know Kenji has written a few articles about how meat changes over different sous vide cooking durations, but I also wonder if the length of time changes the way fat renders somehow. I don't think it's been well studied. Searching the internet reveals I'm not the only person who has noticed this, but some people do not notice it, so there's not really a consensus on what's going on.