just some names to keep things straight:
First jira server (3.10.2) - jira-trackit
Second jiar server (3.12.2) - jira-audit

So jira-trackit has a post function plugin that attempts to connect to jira-audit and create a new ticket. The item will successfully get created on jira-audit however I am unable to retrieve the key of the newly created item from the jira-trackit plugin.

source snip from the jira-trackit plugin:

// Run the create issue code
RemoteIssue returnedIssue = service.createIssue(fToken, rIssue);

final String rIssueKey = returnedIssue.getKey();

LogUtils.getGeneral().info("CreateRemoteIssue::execute: SOAP: new issue has been created " + rIssueKey);
LogUtils.getGeneral().info("CreateRemoteIssue::execute: SOAP: new issue has been created " + returnedIssue.getKey());
LogUtils.getGeneral().info("CreateRemoteIssue::execute: SOAP: new issue has been created project " + returnedIssue.getProject());
LogUtils.getGeneral().info("CreateRemoteIssue::execute: SOAP: new issue has been created priority " + returnedIssue.getPriority());

So again the server.createIssue() works but the returnedIssue.getKey() is always null. It is interesting to note that the other returnedIssue.getProject() and returnedIssue.getPriority() both return real values.

Also, I have a python soap client which will create an issue on jira-audit, this script will successfully retrieve the newly created jira key. So this issue appears to be specific to the java soap interface.