(Late to the party, I know…)
I’ve done a fair amount of sous vide cooking (although it’s been a while since I’ve done beef). I’ve done tri tip and tenderloin a fair number of times but keep my cook times short - probably 2hrs or less most of the time. I’ve made fewer “roast beef” type roasts, but also don’t cook them much longer than needed to bring the roast to bath temp.
I haven’t experimented much with longer times for lean meat and when I have I think it’s always been pork. But, from my limited experience and my recollection from what I’ve read, you’re usually better off keeping the cook time shorter unless you’re primary goal is to tenderize the meat more. Extending the cook time cause the meat to release more moisture, even at low relatively low temps — it’s a lot slower than conventional cooking, but still noticeable. I think if you tried a much shorter cook at the same bath temp, you’d find the roast would seem less well done.
If you haven’t, I recommend reading through some of Kenji’s Food Lab articles where he uses sous vide on Serious Eats. He does a lot of testing, shows the results (including failures), and talks a lot about why the results happen.
What temp was the bath? Also, what kind of machine/setup are you using?
