Going by Cole/Cassandra and Solas/Cass banters, the tranquil state was only for a moment. Although, I have a problem believing that Cassandra was able to empty herself of all emotions for a full year without it, she's way too passionate.
I already brought up the idea that Seekers are resistant/immune to the taint because of their resistance to red lyrium and some people got pissed at me. :/
(If by Taint, you didn't mean the Blight, this entire post is now rendered irrelevant!)
I can hazard a guess at who got pissed at you lol, as they are currently the only guests on my ignore list.
It wouldn't surprise me that they'd be resistant/immune to the taint. Maybe its a benefit of being touched by a Spirit of Faith, or some higher power reason. I mean, Fiona use to be a Warden, and the taint left her for Maker knows what reason, so its not impossible.
Heck, if I remember correctly, and whoever can correct me if I'm wrong, I'd be grateful;
(Spoilerish due to last quest in Act 1 if you side with Mages)
And I like to think it was because of the tranquil state that Cassandra is so passionate. Being tranquil rids the person of their emotions, from the surface to the depths of their souls. So any emotion Cassandra experiences now, there's probably a new depth to how it effects her due to her being tranquil in the past, that hardly any other people are able to achieve.
But who knows, I might be pulling at strings here. But until any and all official answers are given to situations like these, thats the joy of sharing our theories and personal opinions.