I cast Oggok's ultimate just before Malinus's first swing on his first skill. The immortality from Oggok lasts all the way through Call of Lightning, Malinus's second skill. The healing however will proc during Malinus's Call of Lighting cast, even though the immortality is still active. Meaning I'm left with a 1hp tank when I should have the 1hp + the 27k hp from Oggok's immortality wearing off. I've rechallenged Temporal Vortex probably seven or eight times today and it's happened almost every time near the end of the fight.
In the pictures attached you can see how little health my Frurbath has after immortality wears off. Frurbath is geared at around 100,000 hp, and Oggok's ultimate heals for 27k+. In the third picture, you can actually see both that immortality is still active and the 27k healing gained from it supposedly wearing off.