iCloud Photo Library is an absolutely maddening feature. The optimize storage setting seems to be totally useless. I’m sure it does something, but nothing that’s particularly useful for a user. My iPad has 30 GB of space occupied by photos at the moment and other apps are telling me storage is full. There’s no way for me to empty that cache.
In these situations I usually find the only option is to deactivate it and pray that doesn’t break anything. Not particularly user friendly…