I am using Gurobi with callback with a minimization problem. In MIPNODE, I print the upper bound and lower bound. I would expect the lower bound to equal the root relaxation, but instead it starts at 0 and is zero for a long time before it takes on a non-negative value. The upper bound has a value fairly close to the solution from even the first time the MIPNODE callback is entered. Is there any reason the lower bound isn't equal to the root relaxation? I know the root relaxation solution is not zero.