Why a migrated question is still visible to Open Data stack exchange.
For instance this question (link) is on the question list of open data, but if I click on it, I moved to another stack exchange.
The migrated question on the originating site is deleted after 30 days. The stub kept for two reasons:
Migrations can be reversed if the question is closed on the destination site.
It can be confusing to see a question migration when the original is missing. Remember that people on the destination site often won't be able to see deleted questions on the target site.