CT-7567 Rex Survived Nihilus Instakill

Playing 3v3 GAC, using Traya, Savage, and Nihilus against CT Rex, Fives, and Captain Rex (Phoenix). When I used Nihilus's instakill on CT Rex to prevent his max HP attack, he instead recovered to ~35% max HP and Protection like Captain Rex's Unique triggered and prevented the instakill. Is this intended? Fives did not die, and there was no DC on the enemy team.