Jamie Dornan was a guest star. It is also possible that Adelaine Kane did not want to be a regular because she didn’t want the show to have control of her availability. She just came off a long stint on a show, so maybe she didn’t want to get sucked fully into a new show so quickly.
Sean Maguire was a guest star for both Seasons 3 and 4, and yet appeared several times throughout those entire Seasons. So a similar case could happen with Adelaide Kane.