When a show is willing to let an actress like Kiara Barnes (a newcomer) out of her contract for a role on another program, it could say B&B saw that this role would be a great opportunity for her, that they are supportive enough (or thought enough of her as a person) not to lock her into that contract and force her to pass on the role. Maybe they knew (from their plot projections for the next two or three cycles) that her character was not central to any one plotline, and letting her leave would not cause chaos of the writers. As for the lousy exit, I can only guess that she had to leave in a hurry to start pre-production for Fantasy Island and thus did not get a proper send-off. Another option might be that they sense this FI reboot won't last, and by leaving her character's stories unresolved, they can bring her back when/if that show gets cancelled and have built-in reasons for her to step back into the show's stories.
Her contract was up and she decided to not renew it. The news of that broke several months ago, so the writers absolutely had time to wrap up her story in a better way. The fact that they didn't means that they didn't really care enough about Zoe.