Did you already get a chest when you had an S with Zed this season?
Have a look at this;
"You have four chest slots in your profile; once you’ve earned four chests, you won’t be able to earn more until a chest slot unlocks. Chest slots unlock seven days from when you first locked the slot, but only one slot unlocks at a time. Purchasing chests doesn’t contribute to this limit, so even after locking all four slots, you can purchase another. You can purchase up to 25 chests per day.
Once you earn a chest from achieving S rank on a champion, you won’t be able to earn another with that champion until the beginning of the next season. You can see which champions you’ve earned chests with in your profile’s Champions tab. This chest cap resets each season.
If someone in your premade group earns an S rank, you’ll receive a chest as well! The chest you receive is tied to the champion you’re playing, so that champion will be unable to earn more chests for this season. If multiple players in your premade receive an S- or higher in a single game, only one chest will be granted for that game.
You can only earn chests if you're playing a champion you own. So, if you rent a champion or play an unowned champion on the free rotation, you won't be able to earn a chest with them. However, if your premade partner earns an S rank on a champion they don't own, you can still earn a chest from their S rank as long as you own the champion you're playing."
Might shed some light to your question.