Correct. No different to wanting to use a 241 voucher on a trip with non-BA metal in there, or some other BA discount code.

Remember trips are priced from origin to destination essentially, there is sadly no basis for saying how much of a fare is BA and how much it should be discounted, or anything like that.

Logically it is pure and correct the way they are doing it, and I support that it should be done that way, despite the fact it is sometimes annoying.