The problem is due to results being sorted with columns which are not available under Edit Columns.
To resolve without adding back the Customer/Project: Name (Grouped) column, user may do the following:
1. Access the report.
2. Click the Return To Customization button.
3. Click Sorting.
4. Under Choose Sort Order, remove the Customer/Project > Name(Grouped) from the list by clicking the x icon on the right.
5. Preview/Save the Report.
Note : Enhancement 194827 has been filed for this. The issue is due to a feature of pivot reporting engine when generating a summary report. Some of the rows are grouped by column not known to the pivot engine thus they are omitted because the system is considering them as duplicates.