Had the same problem with PartCost on some fields we added a few years back. Our consultant gave us a reason why , that I can’t remember right now. One way we loaded them was through Service Connect and another was BL Tester I believe.
We have a UD field for Character01 on part cost. I looked into DMT to see if we have same issue as you, and we do. My DMT version
The PartCost Character01 field is available in BAQ etc. as your is. I tried to just toss Character01 in (i.e., to make it look like our Part DMT with UD’s in them) for DMT. No go…