This is a known defect planned for a future release (probably VX.2?). But there is a workaround... Use the commandline for mgswrite & mgsread.
NOTE: You need to open a MentorGraphics CMD window from Start > All Programs > <flowversion> > Administrative tools > MGC PCB Command Window <version>
for example: Start > All Programs > PADSVX1.1 > Administrative tools > MGC PCB Command Window VX.1.1.
You can open the MGC PCB Command window and type mgswrite -h (and mgsread -h) for a list of arguments...