OK, but then the output voltage should drop by about 4V when the input power is switched off. I think the plots on your graph must be mixed up - you said green is the output voltage, but that's the only one that doesn't drop. :???:
I'm also still not sure how to compare them. At the bottom of the graphs, it says the red trace is a different scale and offset to the others. If that is the capacitor voltage (as you said) then it shouldn't start higher than the others, but it also shouldn't drop so much.
Could you maybe post plots with everything at the same scale and both axis clearly marked. As is, I can't make out the frequency of the ringing or figure out what any of the voltages are.