When retrieving a list of issues I don't get the spent_hours from every issue. But when I retrieve a single issue I get the spent_hours. Is it possible to do this with a list of issues too?
I would recommend you to make the call to TimeEntry's api https://www.redmine.org/projects/redmine/wiki/Rest_TimeEntries
You will have all the information you need. If you need more information about the issues, you will have to make an additional call to issues as well.