The neck can only influence the patellar reflexes by increasing the response (this due to cord compression and myelopathy) in a condition called hyperreflexia. If the patellar reflex is diminished, this is typically due to compression of the nerve root (new or old), injury to the tendon (prior knee surgery, fracture or tendon rupture) or aging (causes all reflexes to diminish over time).
MS will cause patchy increased reflexes but also a slew of other symptoms. Do not be too concerned with the return of normal reflexes after having increased reflexes and an ACDF for a large cervical hernation. I think that is a good sign.