I am trying to solve a problem similar to employee rostering. The problem I am facing is every time I run the solver, it generates a different assignment. This makes it harder to debug why a particular case was picked over another. Why is this the case?
P.S. My assignment has many hard constraint and all of them may not be satisfied (most cases I still see some negative hard score). So my termination strategy is based on unimprovedSecondsSpentLimit. Could this be the reason?
Yes, it's likely the termination. OptaPlanner's default environmentMode guarantees the exact same solution at the exact same step (*). But CPU cycles differ a lot from run to run, so that means you get more or less steps per run. Use DEBUG logging to see that.
Use stepCountLimit or unimprovedStepCountLimit termination.
(*) Unless specified otherwise in the docs. Simulated Annealing for example will be different even in the exact same step if used with time bound terminations.