Some background on why this happened.
On the home page and certain question listing pages, we display the user and timestamp of the last activity on questions.
Last activity is primarily * any asking, answering, or editing that happens on a question or its answers. When activity occurs, we update two fields on the question record:
LastActivityDate (datetime, null)
LastActivityUserId (FK, int, null)
The issue you reported showed up because, at the time of migration, the question owner did not exist on SU. After seeing the question was migrated, the owner created his SU account and then associated to SO; question ownership was also resolved during account association.
What was not done was setting the
LastActivityUserId during the account association process - we can fix that, but most users with migrated questions aren't as diligent as Uwe in creating new site accounts to tend their migrated questions. So this issue will still persist most of the time.
Thank you for reporting it, though - we can still make migration/association a little bit better :)
* starting a bounty also updates last activity