@JohnDo#98447 This is not a bug imho. The terms and conditions clearly state that each co-owner of a joint account can fully independently make payments (thus af it is any other account they have (access to)). Therefore, it makes sense that the one who made the payment (accepted direct debit) is the one in whose control it is as it was their payment, much like the other co-owner cannot control what transaction you send or control any other of your direct debits.
So indeed, even looking at the terms, this is intended behaviour. :) I believe this setup is actually advantegous and ought not be altered.
-edit- Although I am confused now. The above makes sense, but if I re-read the complaint of OP: even if his partner is the one accepting the direct debit, she cannot edit it...? That would be a bug, she should be the one whose able to control any direct debit of which she was the one to accept it - in accordance with the terms. Just not the ones OP setup. Same should be the case vice-versa.