The n:m relationship input currently does not control value order. Values are sorted by the defined order on the relationship input query, which is relevant during display or when reading out the values via frontend function.
To update the order, you currently have to create any type of order attribute (a whole number for example) and then update that number in a sub form - which you can directly open from the relationship input field. Not very convenient, but the only simple option at the moment.
The only field type that can currently manage sort order, is the Kanban field, though it has a very different use case and is not particular useful for most relationship inputs.
