Same old story with HR and management who have no idea of what it's like to "work the desk" in this part of this industry.
If your roster pattern is fixed, then your time off is yours. Your leave days are just that and anything else is off duty time.
I agree with no sig and I have also know this to be contested in airlines I have worked for but no matter how you try to explain it to them, they can't understand even when you equate it to "day workers" M-F and Sa,Su off.
Keep trying and at least you've got a job.