I have a minimization integer problem. Xpress solver solves it for optimal solution. However, sometimes, the solution is less than the LP relaxed solution. When checking the MIP solution graph, it shows than in the middle of the problem, the best bound suddenly drops lower than the LP relaxed bound and a solution is found in that point. I have been struggling with the issue for a couple of weeks but couldn't find the reason. I would really appreciate any idea or comments on it.