After an email thread is converted to a discussion using Jive for Outlook Plugin and when the discussion is viewed in the Jive Inbox, every message that is part of the email thread and responded by others are displayed as posted by the same person who has converted the email to a discussion.
When the same discussion is viewed on the browser, the names for each message posted will show the correct names of those who are the actual owners of the message. This confirms that the message being rendered by Jive Core in the actual discussion is recorded correctly, but the problem is only with notifications that appear in the Jive Inbox. This is reproducible on a demo instance.
This issue was initially reported as Customer Defect in Jive for Outlook add-on, but was later confirmed to be a bug in Jive core application and resolved in Jive 9.0.7 by Engineering with id JVHOPST-1384.