I would have more sympathy with that because of course that will be the colleagues from the executive of the combined authority agreeing. But
there will still be an issue if it is not possible to appoint a member of the combined authority in whom the mayor has confidence. There are some practical issues here. The choice is already constrained. If you constrain it still further, that will not enhance accountability and democracy but may just oblige the mayor to rely on informal rather than formal officeholders.