It's not a bug, it's a feature of iCloud Photo Library...
What it's downloading are small thumbnails of all the images and you can download the full size by zooming in on them in the Photo Library (Photos app) - it's not downloading 200GB of photos to your phone, and it manages the space dynamically. It also uploads the new photos you take dynamically to iCloud and presuming you've set the phone to Optimize Device Storage it removes the full size versions as space is required.
iCloud Photo Library FAQ - Apple Support
iCloud Photo Library automatically keeps all your photos and videos in the original, high-resolution version. Follow these steps to choose how you store your photos and videos on your device:
- On your iPhone, iPad, or iPod touch, tap Settings > iCloud > Photos or Settings > Photos & Camera, then select a storage setting.
- On your Mac, click Photos > Preferences > iCloud, then select a storage setting.
If you turn on Optimize [device] Storage, iCloud Photo Library will automatically manage the size of your library on your device, so you can make the most of your device's storage and access more photos than ever. All of your original, full-resolution photos and videos are stored in iCloud while device-size versions are kept on your device. You can download the original photos and videos over Wi-Fi or cellular when you need them.
If you turn on Download Originals, iCloud Photo Library will keep your original, full-resolution photos and videos in iCloud and on your device. Download Originals is the default setting for iOS devices with the free 5 GB storage plan and for all Mac devices.