I’m seeing the same. I’ve had a 2016 server fail with INACCESSIBLE_BOOT_DEVICE after applying the April Delta followed by April Cumulative with no reboot in between. Then I rolled back a VM snapshot of the machine, repeated, and did not have the failure.
Microsoft posted a blog as well that this is A Bad Thing
Prevent deployment of Delta and Cumulative updates in the same month
Since Delta update and Cumulative update are available at the same time, it’s important to understand what happens if you deploy both updates in the same month.
If you approve and deploy the same version of the Delta and Cumulative update, you will not only generate additional network traffic since both will be downloaded to the PC, but you may not be able to reboot your computer to Windows after restart.
If both Delta and Cumulative updates are inadvertently installed and your computer is no longer booting, you can recover with the following steps:
Boot into WinRE command prompt
Ideally Microsoft would have handled the case already but clearly they’re not. I think BigFix should do … something. The only (easy) thing that comes to mind is to make the Cumulative Update non-relevant if the Delta Update is pending restart.