What happens when Zilean's ally levels while he's in his passive animation?

Something weird happened to me as I was playing Zilean support. My adc and I were both in bot lane and we both went back at level 6. Theoretically, we should have had approximately the same xp at that point. But he was much faster at buying than me, and when I finished my purchases, he was half way down the lane. Before I got there, he managed to get a kill and some minions and my passive popped up so I activated it. I saw the levelling up animation, but I I remember it being a bit off. Then when I looked at him, he was level 8 all of a sudden and I was only a little ways into level 6. While it's entirely possible, he just had enough xp to be near level 7 when he went back even though I was only a small out into level 6 and then came back and got enough xp to get to near level 8 before I got there, I'm starting to wonder if there's a possible glitch in Zilean's passive

So I want to ask if anybody else has any experience with your allying levelling in the middle of you using your Zilean passive on them and what happens. Cause his passive only works when it can push them up a level, I'm wondering if it may just up them to the next level. Then when they level in the middle of your animation, when the animation is finishes, could it level them again? Anybody knows what happens in this situation? It may have just been a coincidence that I got Graves to 8 instead of 7, but if it's an actual bug, I want to report it.