I think you're right about doctors (or Trainee Healers, which is all I've had so far) in general. Like everyone else, when there's nothing to do for their preference, they choose randomly among their most recent work, their other skills, and non-work. But particularly in a small tribe, healers more often don't have any preferred work available!

However, the boy mentioned above is now the nursery school teacher, and WAY more faithful to that position than either of his two predecessors. He's still check-boxed for Healing, and I think the same choice mechanism explains why that is!