Total pay-cut for on last 6 months shift allowances due to system error

We have been working on shift for over a year now. Before starting shifts, our manager showed us a screenshot of the policy showing that the 1.30 pm to 11 pm shifts were legit. But according to the real policy shifts must overlap between 11 pm and 6 am. The part my manager mentioned was actually a part of a different portion of the policy non-applicable to the current scenario. Now there was an error in the system portal that allowed my manager to assign a shift for us from 1.30 pm to 11 pm, which is not in sync with the real policy and should be available there. If the shifts were from 2 pm to 11:30 pm, it will be aligned with the policy but the current shift assignment was not. Around 6-7 months passed and my manager continues to assign the same 1.30 pm to 11 pm shift and we were also receiving the compensation accordingly. But then on Sep-28, we received an email stating that there was an error in shift assignment, and shift that was not according to the policy will be adjusted in the next payment cycle. So, on the Sept-22 salary, the amount mentioned in the email was deducted. It was a huge amount as we worked for around 50+ shifts for the past 6 months. Now the question is we were kept in the dark about all the policy-related issues. How do we suppose to know that the shift our manager was assigning was not legit? Even so, Why there was communication so late after 6-7 months when we already worked on the shifts? We worked 50+ shifts, and get paid for the same, and then after 6-7 months my employer is coming and stripped us of the whole compensation showing a policy violation. Kindly suggest what action should I follow as an Employee. I have already contacted my manager, and HR person but every time I am getting the policy violation as the reason and they are moving on. Thank You.