Apps · Offloading and restoring apps
Why did Storage Studio leave some of an app's data on my Mac after offloading?
Short answer: A few items are protected by macOS, so Storage Studio leaves them where they are and offloads everything else, telling you when it finishes. Restore still works normally.
macOS protects some per-app locations, and Storage Studio won't force its way past that. When a few protected items can't be archived, it offloads the app and everything else, leaves those items exactly where they were, and tells you in the finish message.
The app still restores correctly — the items it left were never removed.