Multiple related albums exists, but API only returns one

[rsouthward]

I've written an app which lets me to flag certain albums as 'featured' within my app, which has been working fine, but I noticed today that some albums which I was sure I had flagged had lost their flag, so I looked into it a bit further. 

After pulling the albums back from Spotify (using the /aritsts/xxx/albums/ API) I still had some flags left over, which I couldn't attach to an album, an example is..

https://open.spotify.com/album/68wQRlCBGhBW3BNuXKr36W  (this is the ID that I stored months ago)

https://open.spotify.com/album/097eYvf9NKjFnv4xA9s2oV  (this is the ID that's returned when pulling back a list of artists' albums from the API)

https://open.spotify.com/album/0E4xv5gPjykrwBgBZzI8XG  (this is the ID of the album that's shown within Spotify in the list of albums)

When I open any of these links, it shows a little dropdown box at the bottom of the list of tracks saying "2 More Releases" which allows me to choose any of the above, which makes sense.

So I guess my question is, how do I find a list of 'associated albums', it would be great if a list of 'related releases' could be included in the album API definition, or if there was a separate API call to say "Get me a list of linked albums for a artist" then at least I could use this information to update my featured flag. I just had a quick look at `external_ids.upc` to see if this was the same, but all 3 of the above have different UPCs.