I Think My Workplace Hasn't Been Paying Me For My On-duty Lunches...

Location: California

I've been working for this retail chain as a senior assistant manager for about a year now, and this is something that I remembered recently and only just started looking more into. At least one manager is required to be on duty at all times, so when there's only one manager in the store and we need to lunch break, we clock out with a specific option called a "no-management lunch" (will shorten to NM lunch) where we need to stay in the back of the store and tend to any manager-only register functions that pop up during that time. This includes checking bills, doing exchanges/returns, and voiding transactions if necessary. Essentially we are on standby during our lunches.

Now here's the thing. When I first started working here I was told that our NM lunches are paid, but when I started submitting payroll after a few weeks of starting, I noticed that the managers' timecards always day that our NM lunches fall under the "unpaid" category. So last week I decided to do a little digging, and I think I might have found something.

California labor laws state that any lunch where the employee is not /fully/ relieved of their duties is considered an "on-duty" lunch, and therefore they need to be compensated with an hour's pay for every 30 minutes of lunchtime. I cross-referenced this information with my paystubs for the past few paychecks and saw that there are discrepancies based on my expected pay versus my actual pay, even when taking taxes into account. I even looked at the company policy document for any information on meal breaks, and found a section that says that in the event that there is not enough coverage for a manager to take their unpaid lunch outside of the store, they must stay in the backroom, and that they must remain on the clock so that they can be available if needed.

Given all of this information, what should I do? Is this enough for me to file a claim? Should I keep looking for something more concrete than this? Any advice helps.

Tl;dr: I think my workplace isn't paying me properly for my on-duty meal breaks. What should I do?