When we transferred content from Jive to Verint we did our best to match content to usernames.
We also had a limit on how many usernames we could transfer to the new site.
Where we either couldn't identify the username on Verint, or we didn't transfer the username, the content is attributed to 'migrated-user'.
If you discover any of your content in this state, I'm not sure there's a lot we can do about it, but you can try asking in Feedback and Support