Hello @AUnicornWithNoLife, this reads like an issue with the Steam client instead of SteamOS as a platform, so I've transferred this issue report to the steam-for-linux issue tracker.
I had the same problem with a game, today.
For shader cache, disk usage will always be 0.
Nothing extracted yet.
Your system information
Please describe your issue in as much detail as possible:
Describe what you expected should happen and what did happen.
Once all data for game has been downloaded, download page shows 0bps of disk usage (plus rising ETA), however data is still installed, as update / install completes after an amount of time, and game is fully playable
Steps for reproducing this issue: