Wawa:
They are self-healing when connected across the mains.
But not with a small current through them as in a capacitive supply.
I don't know what that would mean, physically. In both cases, no actual charge has to tunnel over the capacitor.
BTW, all datasheets I linked ARE for film capacitors. Not 630V, but that is only about the number, not the letter, AFAIK. (and for 1, we are taking about MUCH higher voltages).