OnePlan administrative

I requested authorization for a procedure at Mulbarton Hospital for 11 March 2022, as the protocol requires me to send quotations from the doctors and hospital, I did so and was issued a GOP (Guarantee of payment ) to the amount of R50000.00. Since I had a similar procedure November 19 2021, I was aware of the clause stating that both procedures would be treated as one visit. Yet despite this the admissions did not pick this up and instead issued the GOP as mentioned above, The hospital, doctors and I believed that this was good despite the clause mentioned. After the surgery OnePlan now stuck to the clause and refused to pay any amount over R50000.00. A staff member admitted that this was a mistake on their part ( I have this in writing - emailed). The GOP is a legal document and I was required to sign its acceptance ( I have a copy ) so why now despite their error am I liable?