When a question is migrated to another site, all answers to it are deleted (its a soft delete - 10k users and mods can still see it) and moved with the question to its target site. The question is then locked by the Community user to prevent other state changing things (votes and edits).
If the migration is not rejected (closed on the target site), in 30 days the question will be deleted on the origin site. Until then, it acts as a redirect.
If the migration is rejected by closing it as something other than a duplicate, the following things happen:
- The answers that came from the origin site are deleted from the question on the target site
- The question on the target site is locked
- The answers on the origin site are undeleted.
All that said, it doesn't appear that you answered it here:
Thus, P.SE never had reason to say there was one answer.
Only in a very few cases does one SE site query another one (migrations to see if the question can be posted there (tags, question ban)).
Putting a "check how many answers are on the target site" can cause some disproportionate amounts of activity on the database, especially when things could move from SU to SO and then having a significant uptick in the "how many answers are on this question over there".
This could have very significant impacts depending on how Stack Exchange partitions the databases or machines.