Activity report doesn't properly filter by assignee/target/source name

If you run an Activity Report and just filter by Target Name using "Contains" and the value "J" (it doesn't have to be J...) then you seem to get all activities in which the target name contains the letter "J" OR doesn't have a target at all.

I would expect it to only shows activities with a target that contains J.

I took a look at the report and can see why it's happening (a temp table is populated with the target, source and assignee activities using separate where clauses) but am not sure it can be fixed without seriously re-factoring the report.