Then arguably it's fortunate that the spell doesn't work incredibly well!
 
It lands. It gets...a substantial amount...of the most acute-injury-like damage, including everything that's imminently life-threatening. But it's...not just that the metaphorical 'cup' for holding life-force is too small to capture all that positive energy, it's also still badly cracked enough that most of the remaining energy doesn't even have a chance to do anything before it spills away. 
The single most important part is that, three seconds ago, Leareth's heart had an electrical rhythm but was having a bit of a time actually moving blood. There was an enormous amount of direct injury to the heart muscle, and Stabilize somehow got it to beat anyway but not well. The Cure gets...some amount...of that. Not nearly all, less than half, even of the damage that looks like the same 'kind' to Shavri, maybe because the untargetable-by-a-Cure damage meant he couldn't absorb enough of the positive energy. But his heart is beating and seems like it's going to keep doing that!
It's not a normal electrical rhythm. The spell seemed to only target some of the Leareth's-body-poisoning-itself problem, and of that it only hit a fraction, and meanwhile the unaddressed injury means that the electrical conduction in his heart would be screwy even with perfect electrolytes and blood pH. He's still in some kind of horrible wide-complex ventricular escape rhythm, just...a surprisingly functional one. He has a blood pressure! They can get a pulse oximetry reading! Neither of them are incredibly delightful numbers but they are, at least, "this patient is alive" numbers. 
He doesn't move or show any sign of waking up.