There are always 4 arguments on stack when recv_internal is called.

By declaring recv_internal with less than 4 arguments you force FunC to ignore most deep variables (they still will be there, but your code will be unaware for the whole TVM execution).

in_msg_full - cell with raw message cell that contains all the flags and additional fields

in_msg_body - slice that only contain body of the message

Please check https://ton.org/docs/learn/tvm-instructions/tvm-overview#initialization-of-tvm for more informations.