Photos storage on iPhone
Yes, your iPhone will remove originals that you have not browsed recently, and then re-download them when you next look at them. If you want to learn more about it, lookup a cache. With the new Photos app, the originals are stored in the cloud, and the images on your phone are just a cache. (Except that when you edit a photo in the cache, Apple preserves both the original and the edits in the cloud.)