I think to cure tranquillity you have to be touched in your mind by a spirit/demon through the veil. There were plenty of tranquil wandering around the White Spire at the same time as Cole without him having any affect on them. Admittedly he'd never actually touched them/connected to their minds. Pharamond seemed to think that was all that was required and that he had been cured before becoming possessed. Then Rhys formulated the idea that if all it required was a demon making contact with the mind, presumably you could do the same with a spirit, provided you could get it interested.
Karl seemed slightly different, although it was definitely contact with Justice that temporarily restored his faculties. The fact that it was temporary indicates that this wasn't the same thing that happened with Pharamond. So I'd guess that if a tranquil was standing very close to a Fade tear they would temporarily recover their normal mind but if they moved away/it was sealed, they would lose it again. However, if they attracted the attention of one of the Fade creatures pouring through and it touched their mind, the restoration would be permanent. The reason this doesn't occur automatically is because tranquil aren't usually attractive to Fade creatures because of their lack of emotion, so they/someone would have to go out of their way to attract them.
Don't ask me why there is this difference; I am only going by the way it has been portrayed in DA2 and Asunder. If Rhys isn't killed at the beginning, may be we will run into him and he will explain further.