When one's rep on a site drops because a question was migrated to another site, that drop is not reflected at all on the user's rep graph or list of changes to rep; it is as if the question were asked on the second site to begin with; all history events and changes associated with it are listed in the history timeline on the second site.
This isn't really accurate, and could cause some confusion especially if any claims or observations were made about rep levels or privileges, because those statements post-migration appear to have been untrue at the time they were made. (Example here.)
Should rep graphs & histories accurately reflect changes to a person's rep on the site the graph/history is for?
Please focus discussion on what would be best from a UX perspective, instead of arguing "no changes from the status quo should be made because that would require some technical work and that's hard."