The project task Actual start date and Actual end date are not being updated when the change request Actual start and Actual end fields are updated. The Project Task Link record is updated as the Change request is updated but the Project task doesn't get updated.
The issue noted here is addressed and fixed by PRB754455 in Madrid, Orlando, New York, and Paris.
If you were provided the workaround Business Rule 'Clear Top Task For Project Task Link', this will cause the issue if you have already upgraded to one of the fixed patches. Before the PRB was fixed, the Business rule was provided as part of a workaround. Therefore, you may disable the 'Clear Top Task For Project Task Link' Business Rule and it will solve the issue (as the fix exists in your system).
*This Business Rule will only exist if you imported it from the ServiceNow via workaround to PRB754455 (This Business Rule does not come with the instance OOB)