The DAC is 0 to Vref (2.5V) in 8bit mode and 0 to Vdd (5V) in 12bit mode. ADC is always 0 to Vref, so obviously you can read only the bottom half of the DAC scale with direct connection, on upper half it just gets saturated.
BTW, if you don't buffer the outputs, except complete mess of any scope readout, the signals are very low power and need to be amplified - any low-to-middle class scope will just introduce too much noise by its own circuits.