When creating a new favorites or fetching the user's list of favorites, the self.url for the favorite itself (not the "item" that was favorited) is incorrect:

{
    "_class": "io.jenkins.blueocean.service.embedded.rest.FavoriteImpl",
    "_links": {
        "self": {
            "_class": "io.jenkins.blueocean.rest.hal.Link",
            "href": "/blue/rest/users/cmeyers/favorites/blueocean-pr-testing/"
        }
    },
    "item": {
        ...
    }

the href should instead be:
/blue/rest/organizations/jenkins/pipelines/blueocean-pr-testing/favorite/

This will allow the UI to use the URL above when we want to remove a favorite by making a PUT request to that URL with this payload:

{"favorite": false}