Fnb - how can I trust you?

After querying my amount due on my credit card last week, since there was a rate cut on the 19th of March of 1 percentage point, and no clear answer was given to me, except that there is dates the cards close (no specifics there). So surely as the rate cut happened mid month, I cannot be held responsible for the higher interest rate - month runs say from 1st to 31st - interest should be on rate from 1st to 19th, and then calculated from 20th-31st on lower interest rate? I haven’t seen any communication on this, in actual fact, the amount due by me end April 2020 is exactly the same amount (perhaps higher) than my repayment this month. How does this work? So no rebate for March, and same amount in April - and please don’t underestimate my intelligence saying it has been app****, as the amounts are the same as before March. And please don’t get someone to call me, as I’m tired of your arrogant, *********** staff. I need a proper explanation in writing to my email address on record, to refer to the bank ombud if needed.