If a civicase is reopened why isn't the Case Coordinator (and other roles) reinstated?
A JIRA issue is now filed at https://issues.civicrm.org/jira/browse/CRM-21815 which provides this change into core.

Patch is at https://github.com/civicrm/civicrm-core/pull/11736 which should be a part of 4.7.32 release.