Try checking the BADI BBP_SAPXML1_OUT_BADI or any other BADI relevant to the document you are trying to transfer
We had a scenario where we wanted to distribute contracts to MDM catalouge. We noticed in the BADI BBP_SAPXML1_OUT_BADI implementation that by defafault only few fields are transferred.
You can create an empty implementation of the BADI and check which fields are getting transferred by default.
As per my understanding you may need to implement the BADI BBP_SAPXML1_OUT_BADI. This also requires changes in the PI