Q:

Why did this rep take a month to disappear?

I pointed out a bug here (10K) which gained 2 up-votes (I think, I'm going by the timeline on Area 51) then was migrated to Area 51 Discussions.  I thought nothing more of it.  
Then today, a month later, I get a -5 for migrated.  I'm sure this is the expected behaviour but I would expect it to happen right away.  
Digging deeper into the question timeline I see that someone upvoted the question on Area 51 Discussions today and it seems to  have been that that prompted the system to take action against my illegal rep stashing. 
Surely the reputation should be removed at the moment of migration now recalcs are dead, and definitely not only after an upvote on the destination site?

A:

Stubs that are left behind after migration are automatically cleared after a month. Yesterday (today, depending on where you are) was exactly a month after your question was migrated and the background task deleted your question. You lose rep only when the post is deleted, not until then.

