We recently closed a question on U&L that was migrated from SF. The OP had no account on U&L and, presumably because they didn't have time to log on before the question was closed, by the time they created one, the question was no longer owned by them.
So, the timeline was that SF user Syntax Error posted their question on SF. SF migrated it to U&L where Syntax Error had no account. It was closed on the target site where it is shown as owned by the non-existent user134378. The OP created an account (user134376) which is not associated with the Syntax Error account and, as a result they could not edit their question, so they posted an answer which was flagged etc.
This should not happen. Automatic account creation on the migration's target site was status-declined already in 2011. As I understand it, this should have been fixed along with the 100 rep bug. So, why is it still happening? Is it because the OP didn't use the same credentials? In any case, this is very bad UX and creates more work for the mods on the target site and is just a pain all around.
Migration of a question should be seamless for the OP. They should always keep ownership of their questions irrespective of which site they end up on.