DC-DC converter isolation capacitance return path?
So basically the transformer inside the module is driven with a high frequency square wave. There is stray capacitance between transformer primary and secondary side. Since all the other capacitances between the output and input are small, the stray capacitance inside the transformer is more than enough to make the square wave that drives the transformer couple to the isolated output that floats, even if there is very little current flow due to the stray capacitance.
When external capacitance is added between input side and the isolated output, that capacitance is far more greater than the stray capacitance of the transformer, so the small stray capacitance and the extremely small current cannot affect much the voltage of the added capacitor, and therefore the square wave signal from driving the transformer has little effect on the output, as it gets back from the output side via the capacitor to input side.