Those lamps have harmonic issues an it's effecting your excitation system. I doubt your VR is a true RMS regulator. A none RMS reg cares less about the peak numbers it's only looking at the bottom numbers an with the amount of distortion on the wave form Mh lights cause, what your seeing is normal. You will find if you add more load or enough unity power factor load your voltage will return to normal settings. Even 252Vac is still a legal voltage.
Once you know how a device, system, ect operates, finding the root cause in the event chain is easy. Until then, you will always be chasing/repairing the effects. Learning is a life time event, if you want to outsmart, whatever item you want to repair.