Edited for clarification:
The problem I had with 2018-3 and 2018-2 windows 10 cumulative updates was that the update would download, it would install, I would restart the computer as directed, but after that restart, Windows update status continued to indicate that a restart was required to install the update. Selecting power option after multiple restarts would keep showing choices "Update and restart" and "update and shutdown". The solution below has stopped the "you need to restart your computer to install the update" situations for both Feb and Mar 2018 cumulative updates.
The reply a month ago by Paul Sey fixed this month's update problem for me. It also fixed the 2018-2 cumulative update problem.
Link to his solution is here:
You run the SC command as he describes, then restart.
No need for clean install.
Why this problem is occurring two months in a row is a different matter.