I'm pretty sure only the first full feeding is fatal.
That's a definitional issue.
A 'full' feeding is, by definition, almost universally fatal. The first instance of such is no exception, but it's also no an exception to the '
almost'.
Only with time, dedication, and practice (typically including at least a few casualties) can a WCV even hope to learn the control it takes to cut short what would otherwise be a 'full' feeding, and many never achieve that (though few probably put in the effort; it's not a popular thing to do).
Some unusual circumstances might greatly assist the WCV, or take the matter entirely out of their hands, however.
As mentioned, a prospective victim might simply possess so much life force that the demon can have its fill without killing.
Alternatively, the act of feeding might simultaneously be the final piece that seals True Love between the WCV and their meal, cutting the meal short at the last possible moment and leaving the succulent morsel on the verge of death's door, but not
quite gone.