Why the edit was not reflected in migrated question?
Reflecting the edits in the source site (the site where the migration has been done) doesn't make sense, as the question in the source site is deleted, at some time.
Judging from the screenshot, taken from Drupal Answers, that happens a month after the question is migrated.
For the reason I said before, synchronizing the questions would require code that, IMO, it is not worth the time spent to write it. It should be considered that most of the users are taken to the question on the destination site, when clicking (on the source site) on a link to a migrated question. This is what happens when you click on Accessing the internal phone memory by swapping out the SIM card. Only moderators on the source site are not automatically directed to the destination site; if I click on How do I use require_once()? I am not redirected to Stack Overflow, but a user that is not moderator would be automatically redirected, if the link doesn't include "?noredirect=1" at the end of the link.
It is still possible to click on a link to get back to the question posted on the source site, but at that point the users have already seen the updated question, and its answers; it would not make sense to reflect the changes in the source site.