There is some issue with MS Solver Foundation’s QP. First I thought it was related to a reported problem about not reporting the constant term in objective values. But this must be something else:
The MSF people reported they know about the problem and it will be fixed for the next release. A workaround is to simplify the objective to obj->x*y-x-y+1. In that case the objective value is not reporting the constant term, but the levels of the variables x and y are correct.