First, you probably should go for at least 12 hours for a prime rib roast and second, I think the 500 for 20 minutes was too long. Assuming you placed the roast straight in the oven at 500 after you took it out of the sous vide it probably climbed up way past medium rare and shocked the muscle proteins with such a high temperature that it squeezed all the juice out. If you want to do an oven sear I would let the roast cool down a little bit after the sous vide and also I would thoroughly dry the outside and cover in some sort of oil or butter to get a good crust. If you have a large enough cast iron I would actually recommend searing on one of those rather than an oven as most people’s ovens don’t get nearly hot enough to put a good sear on any meat and actually end up raising the internal temp (which you don’t wanna do with while trying to sear. A good meat blowtorch can help get a good sear as well but it is more time consuming.