Redirection isn’t fully controlled compared to handover procedure where old and new connecting parties negotiate and gracefully move session. Once the UE is informed (RRC Redirect) to go to other technology, existing connection is terminated and no success/failure is exchanged between controllers (MME and AMF in this case).
Then what would be the way for me to make sure that the remaining data at the CU PDCP layer is to the new eNB/EPS when the UE moves as a result of fallback?
I was assuming that the AMF would receive some form of confirmation from the AMF, and consequently AMF would pass that message to CU-CP via NGAP:UE Context Release Command. After which the CU-UP would be allowed to forward the data the new connecting EPC/eNB.