By Default ARTMAS idoc is not configured to capture WRF_MATGRP field changes. I think you would need to activate this message type to capture inserts intable WRF_MATGRP_SKU . The confusing part is that changes are getting captured.
May be you could try to activate change pointers WRF_MATGRP idoc and see if it works.
Also, WRF_MATGRP_OBJ_WRITE_DOCUMENT is the update function module which SAP uses to insert data/update data in BDCP2 table.