So this has happened twice now, so I'm sure it wasn't just a way a certain enemy reacts to it.

SPOILERS

At the National Guard Training Yard, I was fighting a legendary ghoul when I used a stored crit on it in VATS to finish it off, when instead of killing it, it healed it from about 20% to full health. This happened again later that day on a Super Mutant Brute. Very irritating, since it happened twice on difficult enemies using a laser weapon, which is hard to find ammo for after the first BoS mission.