This may occur when the report's "Show transaction's open as of" date does not yet overlap with the reversal post date of the invoice line item. 

The invoice line item will no longer appear once the Open as of date is on or greater than the reversal post date of the invoice line item that was deleted.

Note: If the reversal has not yet been posted and is editable, it would be possible to open the invoice line item from the Client activity tab and edit the reversal post date This is possible as long as you are marking to show invoice/return line item detail at the bottom of the Client Activity tab, and have your options set to show deleted items.