My best guess is that this may be a bug fixed recently and you should upgrade. But, you also could look in your logs (dspace.log and solr logs) to ensure no errors are occurring. As you note though, this works properly on our
demo.dspace.org site, so it doesn't seem to be a bug in DSpace (at least not in the latest version). It is also possible this bug is fixed only in the upcoming 7.6.2 release (due in July/Aug)... it's not clear to me whether this was fixed recently or a while ago (as I don't see a related bug ticket).