I was trying to use the Apple Clips app the other day to make a quick movie from some short ~10 second videos I had shot on my iPhone 6s an hour earlier. I went to the Library tab, tapped on one of the videos and it popped up the "Downloading from iCloud..." progress bar. It seemed odd that it had to go to the cloud to for such a recent video. I was really curious so I went to the Photos app, went to a different video and tapped on the edit button to see if the video was stored locally, but it wasn't as I saw the little progress circle showing it downloading.
My iPhone has the "Optimize Storage" setting turned on for iCloud Photo Library and I have over 22GB of space available. Is this a bug in iOS 11 or has something changed in how the "Optimize Storage" behavior?