Honouring of termination agreement by Momentumm Health

I terminated my membership effective 31st December 2023 since I was moving to a new medical aid in January. A membership certificate was issued to that effect by Momentum Health confirming my medical aid cover until the 31st December 2023. I had a procedure on the 15th of December which was preauthorised and contacted the medical aid on the 22nd of December to see if my claim was received. On the same day, I received another membership certificate stating my termination date as being the 30th of November. I also received an email rejecting my surgeon's claim stating that the claim would not be paid on the 23rd which alerted me that something was wrong with my membership. I contacted the medical aid today since I have started experiencing complications from the surgery and might have to go back to the hospital if it worsens. Momentum's client services line, emergency hospital preauth stated that I am no longer a member and recommended I go to a government hospital or wait until the 27th when client services are back since they do not have access to the system where they can check on the issue of early termination. Despite signing the termination agreement for the 31st of December and receiving a preauthorisation for my surgery on the 15th December, Momentum has changed the agreement. Is this allowed or legal? Should I not be covered? If I complicate further and the situation becomes life threatening, will Momentum take responsibility since I do not have access to the care that has been agreed upon? How are they just able to change a legal agreement without acceptance from the client?