Update : After deep troubleshooting , it seemed like i found the case of the problem but not able to solve it , what i found is that in the resource pool file some resources have different calendar , those are the ones who make ms project change the resources of some tasks randomly ever time i reopen the project plan file.On the other hand , the resources which have the project's standard calendar are not being replaced or changed .

My question is : How to make all resources have the same standard calendar , I tried to change those resources' calendar ( who is causing the problem ) but with no success , because once I close the calendar dialogue after setting their calendar to standard the change is not saved .

I am using ms project 2016 pro , I really need your help on this .

Thank you .