Originally posted as a question, but now is a solution as it was pretty simple..
In a filter showing only Sub-Tasks, you can't see the parent task summary, only the key.
To show the name, add a Custom Script Field with this:
import com.atlassian.jira.issue.Issue
return issue.getParentObject().getSummary()
Name it something like Parent Task Summary.
Then you just have to add this as a column in your filter (or as a field in a Sub Task screen) and it will show you the parent task summary.