Actually.. I'm not entirely sure that this problem is caused by the November Windows 10 update.
Perhaps that aggravates a problem which really goes back some way in Windows 7. I have to continue using Windows 7 sometimes, as we have customers still using it, and, today, I was reminded that Explorer would often hide the Search box in that OS, too - it
completely disappears, not just disabled, but each Explorer window is [un]affected individually. I have Explorer running 'separate processes' on both OSs. Closing the faulty one on Windows 7, and opening a new one suffices there. For a while.
I have also seen the Windows 10 Search box get grayed as I tabbed to that window (files had just been added to the folder), and then un-gray itself a few seconds later.
I had to back-out the most recent update, as, on restart, it had cleared my desktop of everything except the Recycle bin and Edge, and failed to reopen the folders or run the startup apps. No way I was going to continue with a version that buggy, even if it
does actually fix Search.