In an apprenticeship sense, or in such cases as students learning martial arts, "Master" in the context of a teacher or mentor is also an acceptible term to refer to such a person. In stories like this, apprentices under an accomplished individual or Sages in other fantasy titles can also be referred to as such.
Whatever floats your boat, really. But I don't find anything particularly worrisome about it since this is the established context between Tia and Clerivan.