Controlling LM2596 digitally
Why are you showing the LM2596 "simple switcher" as a 3-terminal regulator? That's extremely misleading.

In the real circuit, the regulator tries to keep its feedback pin at a constant voltage (1.235 V) with respect to ground. If you feed in more voltage from your DAC, then obviously, the regulator's output voltage must be reduced in order to meet that goal.

There's no easy way to make this type of regulator track a control voltage directly. You would need to be able to adjust the reference voltage, and that's hidden inside the chip.