This isn't a bug with 1SE but a well-known industry problem with iCloud Photos. We're trying our best to address it, but it's a moving target.
The best workarounds are to log out and log back into iCloud, download the video locally in your Photos app, or check your device for available storage.
If none of those clear it, tell us through the app's Settings > Contact Support and include the date of the snippet you were trying to use.