Strange behaviour of dc power supply unit- Voltage increases as loading is increased. What could be the reason?
What you are describing is 
Input ESR  [Ω] = ΔV / ΔI  vs. Power Out [W]

The fact that over small range the efficiency jumps and what was a monotonic positive ESR to a negative ESR may or may not be of interest to the design from efficiency, ripple or resonance point of view , but begs the questions about your missing details. 

So what can you define about the design and test process.?

Input impedance is related to the V² ratio  like turns ratio , N² on a transformer. But there may be more complex mode switching occurring or spurious resonance.

Show your data with Vin/Iin,  ripple vs Output, ripple or whatever spec it is not meeting.