I might be way off as i def do not work in HR..
I've taken mat leave a bunch of times from my govt job. Prior to having kids, every yr, on the anniversary of my start date, i got an incremental pay increase as per the award.
After taking mat leave and returning part time it now takes twice as long to get the incremental pay increase, simply because it takes me a lot longer to reach x hours of work required to be eligible for the increase than it does for someone who works full time.
I see it as they are giving you half the rise rather than making you wait twice as long for the full rise?
I dont think its discrimination as i would assume it would happen to anyone who went from ft to pt work regardless of their reason.
A meeting with your manager should clear it up anyway.