A switching power supply should be regulated. The one you have is defective. Some do have a minimum load, but I’ve never seen that on an adapter that can be unplugged from the load.

A linear power supply has a linear regulator and will supply its rated voltage with very little ripple. The unregulated power supplies are just a transformer, rectifier & capacitor. They only provide their rated voltage when they are supplying their full rated current and there will be a lot of ripple.