I have noticed that healers continue to learn the strange plants by their own only if they are not appeal to do something else. I mean, if "healing" is their most important skill. If they are master scientist (for example) and only adept doctor, they prefer doing any thing else except studying the plants. I had villagers, after that all puzzles are solved, who are only healer and they became master doctors!
I don't know if it is the only explanation but I think it is one of them.