Summoner spell Cleanse not cleansing Zed's Death Mark?

I just played a game in which I picked the summoner spell cleanse. The enemy team had a Zed which tried to assassinate me quite a few times. I was fed and was able to kill him when he attemped to take my life... But then the mark usualy killed me at that point. Now here's the issue. When I tried to cleanse the mark, it didn't work at all. It simply stayed above my head like a guillotine that dropped a second later.

The first time this happened, I though I did something wrong or the game had glitched a bit. But when the incident happened another time, I now had the proof that Cleanse does not remove Zed's Death Mark.

I'm not sure if this is either a bug or an intended feature. Cleanse's description is almost identical to QSS's tooltip, and most player would tell you it does the exact same thing, put aside the 3 sec reduced CC. But this is clearly not true as Zed's ult (and possibly more spells that apply a debuff) bypass Cleanse.

Now I haven't tested all debuffing abilities in the game like poisonning abilities and etc. and whether or not they can't get cleansed, but I'm trying to bring this one to your attention. Common sense would say that Zed's ult should get removed by Cleanse just like QSS removes it.

I'd love to know if this is something that was intended or not. If this is in fact not a bug, can we please get the reasoning behind this decision? And also, if you decide not to "fix" Cleanse, can you add more info on it's tooltip to clearly state that some non CC abilities like Zed's ult bypass it? (for clarity!)

I'll be watching this post regularly and if you need any additional info regarding the settings of this bug feel free to ask me.