After declaring a champion for your Champion/Challenger test, you notice that the test (ex: "Whole Emails" test) in the Marketing Activities asset tree under your Email Asset is missing.
When you declare a champion, you select a Declare Date. Once this date/time is reached, only the new champion will be sent. After the test is over, it is expected behavior that the test (ex: "Whole Emails" test) will no longer be shown in the tree. This is because the test is no longer active. If you would like to retain your test data, it is recommended that you save this outside of Marketo.