Using Googles own "try this api" feature with google photos api. There is a bug with the REST request to add photos to existing albums. Attached a screenshot as proof. I raised this issue here because thats what their support section says to do so please dont downvote. I can add photos directly through the google photos app and thats it.
So i figured out why this is happening.
See "Media items can be created only within the albums created by your app. For more information, see Authentication and authorization scopes." from this link: https://developers.google.com/photos/library/guides/upload-media
Google's api wont let you modify an album that was created through another means. In this case i tried modifying albums which i'd created months ago via the UI. Then i was trying to add to those collections via an OAuth2 token using code, postman and eventually googles own api explorer.
My solution was to, in code, create a new album, get its ID and append images to that instead. That was the only way to get a "writable" google album.