Hi Carlos,
I also faced same error when tried to pass SAP_Origin in Json Payload in case of Create deep entity.
However I found another way of doing this, would you like to try this as below.
/sap/opu/odata/zget_materials_SRV;o=My_001/MaterialSet
Where My_001 is your system alias.
Payload :
{
"Material":"0065",
"Werks":"0025",
"NavArti": [],
"NavValu": []
}
Please check below document if this helps, you can try using System ID also 
.
Exploring Multiple Origin using System ID and Client
Regards,
Tarun