Why do deleted iCloud photos still appear on my devices?

I used to download photo's from iCloud onto my NAS at home, delete the photos in iCloud and it would delete them across my devices freeing up room. Now, it seems if i delete in icloud photos it's not syncing to devices. I'm using an iPhone12Pro. While I get Apple makes the most cumbersome devices, I don't want to increase iCloud storage, I just want to move them to my home NAS and then delete them in iCloud. delete thousands of photos on a phone device is not user friendly.

Does anyone know what happened?

[Re-Titled by Moderator]