From when can builder start calculating CAM charges to flat owners?

Hi, I had booked my flat in Aug 2020, by paying 2L booking amount at the time. As part of the deal, I was offered 18 months maintenance (CAM) free period. During the next couple of months, partial payments amounting to a total 20% of the property cost were made (there was an instance where application was refilled and booking amount refunded and new cheque shared as OC was received by the builder and hence, the same was done to remove the payable GST). Surprisingly, I received Offer of Possession from the builder in Oct 2020, even with only the partial payments done till then. There were other instances where Pre-cancellation notice was received in Dec 2020 as well as Cancellation notice received in Jan 2021, which their CRM team asked us to ignore as they were generated / sent by their system due to pending payments at the time (the payments were pending due to delays in the builder presenting the required documents needed for the home loan which was availed to make the remaining 80% of the payment). After a few delays by the builder in producing required documents for home loan, the full payment was done in Jan 2021 via home loan. The handover took a few more months and finally possession was taken in Aug 2021. Now, I started receiving CAM invoices starting from June 2022, which is when I found out that they had started the 18-month CAM-free period from the issuance of the Offer of Possession way back in Nov 2020, whereas my expectation was that the same will be started from recieving possession of my flat. Kindly shed light on the same as per HARERA what are the rules pertaining to CAM charges and from when the builder can start charging these invoices to the buyer. Many thanks, Sumit