I'm sorry to hear about the missing Japanese characters when viewing the document. We've had a similar report under JIRA bug JIVE-60140 on this behavior with Japanese and Cyrillic characters, but our engineering team has been unable to reproduce the issue. The bug has been closed since we cannot reproduce it.
Due to the complexity of the issue, I would recommend submitting a support case in your private Jive Community customer group for targeted troubleshooting.
I hope this helps. Let me now if you had any other questions.