A teacher of teachers in a field I'm involved in frequently says:
the best teachers are often those who only recently learned the material themselves
The rationale for the statement is that someone who has recently learned the material is more intimately aware of the stumbling blocks someone goes through when learning it, and can therefore be cognisant of those when teaching it to other beginners, thus making them a better teacher.
By contrast, someone who has known the subject matter for many decades may have completely forgotten what it was like when first learning the material, and could be therefore prone to impatience, or underestimating complexity or learning time.
Does this concept have a name? Or is there any study that backs up the idea that more experienced instructors can sometimes be blind to the most basic concepts in their discipline (since they take them for granted), and therefore (paradoxically) make worse teachers than those with less subject matter expertise?