iCloud backup tries to back up my Photo Library, even though I have iCloud on for Photos

Question

I had to give my iPhone in for servicing, so I did a local backup to my Mac, erased all data, and then restored it from the local backup.

All was, well until it said it could not back up to iCloud. The reason was it was out of space as it was trying to back up my "Photo Library":

I'd never seen the "Photo Library" app in iCloud backup.

I've always been using iCloud Photos for syncing my photos, and

^{Click images to open full screen}

Any idea why "Photo Library" suddenly appeared in my iCloud backup and how to fix that?

Thanks.