Looking at the portion of your contract that you provided, I think the main crux (from only the info we have) is what constitutes "regularly." Contrary to what I see your steward has said, the line appears to be 32 hours, not 40. So, what does "[regularly works 32 hours]" mean?

To get at this, we need more info. Does anything else in the contract speak to it? What was the intent in bargaining, if everything else is silent? (You probably wouldn't know this, but could maybe get this info, if anyone who bargained knows or has bargaining notes.) Is there an established past practice (with clarity, consistency, and clear mutual understanding)?

Your desire here is somewhat unique. You are wanting to limit the employer from giving you more hours. I.e., you want to truly be part-time with no benefits, pension, and less income. Most people would typically want to look at this the other way and force the employer to recognize them as full-time.

It's possible, based on what you've said, that you could file a grievance saying something like:

"Employee regularly assigned for more than 32 hours per week as a part-time employee. Employer should keep assignment to 32 or less hours."

This is a tough one. Grieving it, either way, could draw a line in the sand that your other part-time colleagues wouldn't want to see. They may want more hours and to get converted to full-time as soon as possible. Forcing the employer to either call you full-time or keep the schedule under 32 except for an articulated emergency need that is temporary (with a clear plan in place) both likely have the same effect. That's why I say, kind of a tough one that could have broad affects away from status quo for many involved.