To elaborate on the other good answers, this will occur more often on Windows 10 systems that are running relatively low on free disk space. The Windows Update process will automatically compress files and folders to ensure that crucial operating system patches will install successfully:

Compressing files

To help free up disk space, this update may compress files in your user profile directory so that Windows Update can install important updates. When files or folders are compressed, they appear as having two blue arrows overlaid on the icon. Depending on your File Explorer settings, you may see icons that look larger or smaller. The following screen shot shows an example of these icons.

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.

Source:  Update to Windows 10, versions 1507, 1511, 1607, 1703, 1709, and 1803 for update reliability

To elaborate on the potential root cause, this will occur more often on Windows 10 systems that are running relatively low on free disk space. The Windows Update process will automatically compress files and folders to ensure that crucial operating system patches will install successfully:

Compressing files

To help free up disk space, this update may compress files in your user profile directory so that Windows Update can install important updates. When files or folders are compressed, they appear as having two blue arrows overlaid on the icon. Depending on your File Explorer settings, you may see icons that look larger or smaller. The following screen shot shows an example of these icons.

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.

Sources: 
Windows Update - KB4023057 Release Notes
Microsoft trying to push KB4023057 again to user systems