Zilean's ult doesn't always heal for the full amount

Zilean's ult restores exactly 50 HP too little sometimes.

Steps to reproduce: (~50% reproduction rate)

1. Howling Abyss
2. Cast ult on self
3. Die before buff expires

Expected results: Revive with 600/850/1100 + 2.0 * AP hitpoints. Actual results: About half the time, exactly 50 hp is missing. E.g. It should heal for 600, but you actually have 550 hitpoints when you revive. I noticed my hitpoints would (usually) go to the correct amount and then drop by 50 as soon as the lifebar refreshed (0.1 seconds later or whatever).

I didn't see any correlation between actions and this bug -- I would do the same thing with the same timing, and sometimes I'd get my full hp and sometimes I wouldn't.