I 100% confirm that’s not the case, sorry. From experience. I do play Arena exclusively, and I’ve played dozens of runs since the patch. EDIT: I’m also super tired and 100% incorrect, apparently (or was it 99%? ^^). It seems that 8-1 still counts the same as 8-0. So it’s just if you take 2 losses that you get no Seal. My bad.
Also, I’d like to add that this “Perfect Arena Run” requirement comes up right when the Arena is more RNG-based than ever before. There are so many troops and weapons these days that can completely win or lose you the game in one or two (un)lucky rolls… It’s impossible to get 8-0, or even just 8 wins consistently nowadays. It used to be the case a few patches ago, but now it’s come to the point that I actually get crushed by the AI in one run out of 5 or 6 maybe (by this I mean I end up at 2-2 or 3-2, never mind 8-1). Even @Archenassa gets losses regularly.
You can be a great Arena player, make a very solid draft, play your best every single turn, and still get obliterated by things like, say… an unfortunate timing (for you) on ONE SHOT from the opponent’s: Spirit Fox, Dragon Fire, Creeping Death, Succubus, Dragonette, Astral Spirit, Lion Prince, Spark Rocket, the ever-boring Imperial Jewel… to name but a few of the frequent offenders. That’s on top of the usual permanent threat of dying to a truckload of 4 or 5 matches coming from the sky with a shower of skulls out of nowhere. And, you know, just plain bad luck. My Hag might buff my Ogre’s Magic (aka null) while applying useless crap like Stun or Disease to her target, while the opponent’s Hag might buff his top troop’s Attack and Burn or DeathMark my MVP (and the AI’s DeathMark tends to proc immediately, contrary to a human player’s DeathMark ^^). Anything goes, really. There’s absolutely no guarantee that any given Arena game won’t go sour at any point, these days.(*)
So yeah, it might be a tidbit unfair to us Arena players that we get no Seal at all if we take even 1 loss throughout a run. EDIT: if we don’t reach 8 wins. Going 8-1 nowadays is harder than going 8-0 used to be.
(*) Yeah, I’ve been having a bit of a bad run lately… What gave it away?! =)