Decreased interest rate on my investment

I have been investing with African Bank since 2021 through a Tax free investment and all of a sudden I just received an SMS that my interest rate has been reduced with no additional explanation as to why. The reserve bank has not reduced the rate so I don't understand why African Bank has just decided to reduce my interest rate and not even bother to send a letter with the reasons why. My sister has the same investment and her rate was not reduced so I don't understand