If a client makes a POST request to a URL from a relationship link, the server MUST add the specified members to the relationship unless they are already present. If a given type and id is already in the relationship, the server MUST NOT add it again.
Currently this does not work, it treats such requests as a PATCH request and will replace all of the relationships instead of adding to them.
This part of the spec: https://jsonapi.org/format/1.1/#crud-updating-to-many-relationships stipulates:
Currently this does not work, it treats such requests as a PATCH request and will replace all of the relationships instead of adding to them.