How to get all uploads from official artist channels via the YouTube Data API

I am trying to query youtube for all uploads relating to a channel, but for official artist channels such as: https://www.youtube.com/channel/UC-jHioVmLr8Cb1lFBIZD2hQ/

When I try to make the API call to retrieve the "uploads" playlist, it succeeds, but returns an empty playlist.

Finding the uploads playlist via https://www.googleapis.com/youtube/v3/channels?part=contentDetails&id=UC-jHioVmLr8Cb1lFBIZD2hQ (API key omitted) returns

{"kind": "youtube#channelListResponse","etag": "\"XpPGQXPnxQJhLgs6enD_n8JR4Qk/SQbT6DxUQ-PRgTma_VN0xA8OmHE\"","pageInfo": {"totalResults": 1,"resultsPerPage": 1    },"items": [        {"kind": "youtube#channel","etag": "\"XpPGQXPnxQJhLgs6enD_n8JR4Qk/m44G0g4BqX0Af7CaFKoFcqJDqm0\"","id": "UC-jHioVmLr8Cb1lFBIZD2hQ","contentDetails": {"relatedPlaylists": {"uploads": "UU-jHioVmLr8Cb1lFBIZD2hQ","watchHistory": "HL","watchLater": "WL"                }            }        }    ]}

but retrieving the items in the uploads playlist via https://www.googleapis.com/youtube/v3/playlistItems?playlistId=UU-jHioVmLr8Cb1lFBIZD2hQ&part=id,contentDetails&maxResults=50 returns

{"kind": "youtube#playlistItemListResponse","etag": "\"XpPGQXPnxQJhLgs6enD_n8JR4Qk/E0c9LqWjuNHvL-UjCIKS4N9Fn5M\"","pageInfo": {"totalResults": 0,"resultsPerPage": 50    },"items": []}

I would like to be able to just pull the videos that show up from the videos tab, but cannot seem to do that with the API.