The assumption is DTE rejected the voucher because your heating costs are mixed in with all the other electrical usage and does not represent just your heating costs. I do not know what might be acceptable but the heating costs would somehow need to be seperate from all other electrical costs.
You have the right to an appeal if rejected or possibly could return the voucher back to the state with an explanation and request a check. You might hit the same issue with the state.
The attached file has two links that may be helpful.