According to
https://wiki.openstreetmap.org/wiki/Relation:waterway#Members
only water leaving the river and returning to it should be given the side_stream role.

And this tributary hadn't been given the side_stream role:
https://www.openstreetmap.org/way/411049009/history

If you want to undo my edits, here is the changeset:
https://www.openstreetmap.org/changeset/160256322