Hi PCR,
On one of my machines, windows update offered the upgrade from 1809 to 1903 the first day it was available on 5/21/2019, but not on any of the other ones.
Since that worked fine, I decided to upgrade all the other machines by downloading the 1903 installation ISO using the media creator 1903 available on the MS site.
That ISO turned out to be build 18362.30.
Anyway when I tried upgrading from the USB that I created it was blocked in every machine including the one that was already at 1903 for the reasons explained in this link:
https://support.microsoft.com/en-us/help/4500988/windows-update-hold-for-windows-10
I then decided to do the upgrades directly from the mounted ISO file residing on an internal local drive and everything worked fine.
After that, windows update downloaded all the fixes to bring up the machines to build 18362.145. (KB4497935)
Now I was just trying to see if they also fixed the Windows installer to work from the USB stick, or when any USB disk is plugged in, but clearly they did not, at least not yet.