I can say that I don’t remember ever seeing a pars fracture at L5 or L4 from a microdiscectomy but I assume it can happen. The higher levels have a much more narrow pars and I do believe I have seen three or four. That is however out of thousands (or tens of thousands) that I have seen in my office. If there is a foraminotomy however, this does thin down the pars to try and clear the foramen and there is a small percentage that pars fractures can occasionally occur.
A one level fusion normally does not cause significant stress to the levels around but there is some additional stress. Most of the problems are due to genetics and not the fusion.
The surgeon might not have noticed the pars fracture as it might not have been present at the time of surgery. This fracture would most likely happen later with the patient undergoing some extension activity and the additional stress causing the fracture.