We are on CA PPM 15.4. We do status reports in the New UX every two weeks. Sometimes, the Completed Tasks section is not accurate. It shows the same task two reports in a row or it doesn't show all the tasks that have been completed since the last report. We checked the "last updated" field to verify when it was last updated. We completed tasks right before creating the report. Both people that have experienced this were using the New UX to check off that the task was completed.
The OOTB status report is using data in the Data Warehouse. Given that your PM's are 'completing tasks right before creating the report', the Data Warehouse may not have been refreshed, prior to running the report. Either run "Load Data Warehouse" more regularly (try in non-prod environment first), or, create a custom version of the status report pointed the live data (eg using Data Source CA PPM BEAN). I generally go with the second option.