Known bug. Import the address instead of sweeping. Do a Help -> Rescan Balance once the import is over (should take <1 min). Once you get the balance, spend the coins manually.
Be careful about adding it to your usual wallet, though. In principle you could accidentally ask to receive money to that address sometime in the future, and it may be known to other parties, and is not covered by the usual backup (as it was not created by the seed). Also, spending the coins from exactly this address may require using coin control. It may be easier to create a new wallet, import it into the new wallet, spend everything from the new wallet to an address in the old wallet, and then remove the old wallet.