Error in 5G SA: Message Type Not Compatible With the Protocol State

Hello Experts,

  1. UE registers with SA NR cell.
  2. UE initiates call and does EPS fallback to LTE cell.
  3. While call is active, UE receives SMS message through the LTE cell.
  4. UE doesn’t respond with CP-ACK and an error is seen. Error is ‘message type not compatible with protocol state’.

What could be issue here?