Seems strange to me. What initially sprang to mind me was the vacant possession thing which begs the question whether the buyers have a buy to let mortgage or have consent to let.
Usually lenders would need to ok a tenancy agreement to protect themselves (or have their lawyers, usually the same one as the buyer) certify it is ok to protect them.
Contracts can be varied with consent of both sides. I wonder if it is mortgage related. Maybe they have an issue with the current tenancy agreement or for some reason they haven't been told (maybe late stage when they have already exchanged and it might cause issues with the lender raised at this point - issues with the lender could, worst case scenario, delay mortgage funds or result in withdrawal of a mortgage offer.
Anything which is a means of covering up a true situation is not a good thing and could lead to future issues.
I'm not saying that is the case, of course.