It appears that the following sequence results in orphaned answers:
- Question is migrated from A.SE to B.SE.
- An answer is posted on B.SE.
- Question is closed on B.SE, which causes a "migration-rejection," effectively putting it back on A.SE.
The weird thing is that answers posted on A.SE follow the migration to B.SE and are retained when the migration is rejected and the question reverts to A.SE. But answers posted on B.SE do not follow the question back to A.SE.
I'm not sure what the final disposition of these questions and answers are, especially if the question is left closed on A.SE. But it appears that the answers posted while the question was on B.SE are going to be lost, in which case this seems like a bug.