When the Actual End Date of a Project or Project Task is updated, the Planned End Date is set to the Actual End Date. The Planned End Date gets overwritten with the Actual End Date as well when the State is moved to Closed.
Steps to Reproduce
1. Create a project for some past date.
2. Add a project task.
3. Move the project task to work in progress.
4. Close the project task.
Notice the planned end date becomes the same as the actual end date.
This problem is fixed in all currently supported releases. Please review the Fixed In section to determine the latest version with a permanent fix your instance can be upgraded to.
If you notice this happening in an already upgraded instance, check all the active Business Rules called "Recalculate" on the [planned_task] table. There should be only one active. If there are two of these Business Rules active instead, then disable the one with sys_id=69d578a07f000001353c4187f328c586.
Related Problem: PRB1175271