I noticed same behaviour importing multiple keys in a row into the QT wallet. There must be a bug in the QT client.

You have to keep the timely order (according to "timereceived" in listtransactions) to get the correct coin generation timestamp shown in QT client, i.e. first import the key with the oldest coins, then the second oldest etc.

If you import a key with newer coins first and then one with older coins, the row with address of the older coins will show the timestamp of the newer coins, as it did in your screenshot.