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There is one known case where Windows will compress your User folders. That is when windows update is preparing to receive a Features Update. If you are short on space your User folder may be compressed, then uncompressed after the update has installed successfully. This can happen automatically and is by design. You free space fluctuating suggests windows update may be downloading in the background, then when space is short more user folders get compressed.
KB4023057 Update to Windows 10 for update reliability - November 15 - Windows 10 ForumsTo help free up disk space, this update may compress files in your user profile directory so that Windows Update can install important updates...
...After you install the update, your files are restored to their original state, and the blue arrows disappear from the file icons in File Explorer. At any point during the update process, you should be able to access your files.
What do you see in Settings > Update & Security > Windows update? Any pending updates?