How dumpprivkey/importprivkey works? Don't works in usual way.
Please get the 'shorten' address from Coin Control dialog (Preference > Display > Display coin control features).
Check in the send tab you'll see that there is 'Inputs..." button, inside you should see the list of all true addresses for your wallet which you can use with dumpprivkey and importprivkey.
Thank you for your support.