Hi Professor Goldsbury,
Actually, it's just a matter of convenience. For a number of reasons, such as time constraints and inability to make contact, other instructors cannot be scheduled.
Please help me provide enough information for a reasoned judgment. What questions do you have?
One of the problems in an anonymous forum such as this is that the information given should not allow anyone to know who you are. Which is fine. The down side is that the information given sometimes does not really allow any one to make a reasoned judgment. In this case, one extreme is 'Shut up and train'; 'Find another dojo,' is the other.
The chief instructor is boss in his/her own dojo, but it would be very odd if the chief instructor consistently and systematically passed over all the senior ranks and asked the lowest ranked student to teach. I know that if I did this in my own dojo when I was absent, the members would quickly start looking for another dojo.
Hence my question: if it is of concern to you, have you talked to the instructor? If it is not of any concern, well, I was curious why you bothered to start the thread.