ABSA *****ing my money

Myself and my mother was working for ABSA in 2007 when we app**** for a Home Loan from ABSA, this loan was then approved on a staff interest rate "FBT rate" this rate is the prime lending rate -2.50% as per ABSA employment benefit agreement. The terms and conditions of this benefit states that you only qualify for this rate if you are in the employment of ABSA or on Pension as an ABSA employee or if otherwise documented by ABSA. My mother subsequently then went on pension as an ABSA pensioner and still then qualifies for the benefit going forward. I was then in 2015 transfer to SBV with a SECTION 197 from ABSA. As part of the negotiation process between ABSA, SBV and SASBO the financial union, it was negotiated that ALL current loan agreements in place by the date of transfer will stay on the FBT rate until the end of the loan term. Formal documents to this regard from ABSA will be attach. I notice on 22/05/2024 that the current interest for the joint home loan account 8************ was on 11.75% which is the current prime lending rate and not the FBT rate that must be 11.75% - 2.50% = 9.25%. I then escalated this to ABSA home loans on 22/05/2024, Sam Shabangu, ABSA Customer Experience Management from ABSA home loans then rep**** via mail to me on the 23/05/2024 that the interest was changed back to the FBT rate and that an amount of R297-79 will be credited back to the account due to over charge of the interest. To note is that my mother also in her capacity picked up the issue regarding the wrong interest rate and queried it with ABSA a month or 2 back before my query lodged on 22 May 2024, they then corrected the rate also back to the FBT rate as per the original agreement, after my mother’s query the rate was again changed back to the prime lending rate. This is the reason why only an amount of R297-79 was paid back to the home loan account from my query lodged on 22 May 2024. I then went through my historically letters received from ABSA on the ABSA banking app regarding interest rate changes that took placed in line with SARB interest hikes, these communications only went back for the last 10 communications to November 2021. After scrutinizing these letters i picked up that all these communications the interest rate was on the prime lending rate and not the FBT rate as per our agreement between ABSA and us. I then escalate this back to Sam from ABSA where I asked him that this must be corrected back to the date that the rate change was changed incorrectly to the prime rate, I also asked him that I want to know when this mistake by ABSA happened as my communication letters only goes back to November 2021. This request was declined by ABSA and Sam stated that no historically mistakes by ABSA will be corrected or the extra interest charge for this period credited back to the home loan account. This is concerning as the mistake made from the 1st query from my mother 2 months ago to when I lodge my query on 22 May 2024 was corrected by ABSA AND interest over calculated WAS paid back to us ???? Why can they take liability for this mistake BUT not the mistake made years ago ???