The Conflict checker finishes running after having saved a change record and the form has already reloaded so the conflict field does not accurately reflect whether there is a conflict or not until you refresh the page again.
Release or Environment
This is the expected behavior based on the OOB configuration of the conflict checker process which dictates that automatically run conflict checking should be run in the background.
You can modify the business rules 'Update Conflicts' in order to prevent the conflict checker from running in the background.
1. Go to the BR 'Update Conflicts'
2. On line 10 where you see 'worker.setBackground(true)'
3. Change 'true' to 'false' and then save the change
4. Test changing the planned start and end dates to confirm that the conflict field now reflects the accurate conflict status of the change request.