How can I exclude the photos that are in iCloud Photo Library from my iPhone’s iTunes backup? When making a backup of my iPhone through iTunes, how can I exclude the photos that are already stored in my iCloud Photo Library from the backup?

I have the impression that an iTunes backup includes all of the iPhone’s data related to iCloud Photo Library. I made a few backups of my iPhone while it is still downloading my iCloud Photo Library (my MacBook is still uploading to it), and the size of the backup keeps increasing. I had also cleared out Photos on my iPhone and made a backup before enabling iCloud Photo Library on it; the size difference of the backups before and after is the same as the amount of storage used by the Photos app on the iPhone (which is several gigabytes).

Photos on my MacBook is set to “Download Originals to this Mac”, while on my iPhone it is set to “Optimize iPhone Storage”.

I don’t really need the iTunes backups of my iPhone to include another copy of my photos (except perhaps for any photos taken on my iPhone which have not yet been uploaded to my iCloud Photo Library); is there any way to exclude them?

I realize this question is very similar to the question on “Omitting photos when backing up an iphone to a macbook with itunes”, but that question doesn’t seem to involve the use of iCloud Photo Library.
After searching some more, I came across the Macworld article “How to exclude your iCloud Photo Library from an iTunes backup”. The article points out that iTunes backups are not supposed to include the iCloud Photo Library data in the first place, as documented by Apple Support:


  An iTunes backup doesn't include:
  
  
  Photos already stored in the cloud, like My Photo Stream, and iCloud Photo Library
  


The article reports that others have experienced the same problem where the backup does include the iCloud Photo Library data, and suggests disabling and re-enabling iCloud Photo Library on the iPhone.

I think I will wait for the upload from my MacBook to finish before trying this. Perhaps the problem will resolve itself after the initial upload is complete. I will edit my answer later.

Edit: The above tip may be helpful to others, though unfortunately I have to report it didn’t resolve the issue for me. Apart from disabling and re-enabling iCloud Photo Library on my iPhone, I have also tried deleting its backup in iTunes before making a new backup. But I find that the iTunes backups are still taking up a few gigabytes of additional space with iCloud Photo Library turned on compared to when it’s turned off. I’m not sure what else to try; other tips, or confirmation from others experiencing the same issue would be welcome.