Issue: When attempting to install macOS 10.14.4 or newer through System Preferences -> Software Updates, the download completes and the machine appears to restart, but then the login screen appears rather quickly. A check of the Apple Menu -> About this Mac shows that the OS has not been updated.
Analysis: Unclear as to why this is occurring.
Resolution: Launch Terminal and use the following command: sudo softwareupdate -iaR after which you will be prompted to enter a system administrator password. macOS will then check for and install any pending software updates.