This is perfectly normal, the atoms have no reason to stay in the original unit cell if the temperature is high enough. Especially in order to track quantities like mean square displacement, you have to let the atoms travel outside the box and not just wrap them by periodic boundary conditions. If you still prefer that for your animation, you can choose it in the Movie Tool buy right-clicking and choosing "Wrap atoms".
Also you can always wrap a structure in the Builder, there is a tool for it under Bulk Tools.