The circuit is from the above link. There is some problem with the control loop. The output voltage doesnt stay constant within the input range of 12-24v for which it is designed. Even for any simple boost/buck circuit I am having the same problem with the control loop.
Boost regulators are notorious for needing a minimum load to keep them in regulation. Think about it a little. At the lowest duty cycle possible the inductor is charged for a very short time and this gathers in energy. Then that energy is released via D2 into the output circuit. You do this maybe 50,000 times per second and that means you are transferring real power to the output but, there's too light a load and the output just keeps rising. The higher your input voltage the more energy per cycle transfers so, when you raise your input supply, the output rises so more.
The 1 kohm (R5) you have may not be enough. Try putting 100 ohm on the output and see what happens.