Low quality iCloud video

All my videos in iCloud are in low-resolution quality. Apple support says "we don't know, that shouldn't happen".

Example: I shoot a video by iPhoneX 4k60fps 40 min. In iCloud it weight ~30mb and looks like 480x360. Please help! I read some advices like a "click download on iCloud, choose "full quality", although I do not have such option in iCloud.