Thanks for the clarifications and sorry that I cannot offer more on the point advice ... maybe somebody else?

From your description, I'm guessing that when running 16.x software you have a DIFFERENT Windows installation when using a 15.x or 16.x VM computer. Thus the real difference is the Windows installation and NOT VM-computer upgrade into 16.x version.

Given those possibilities:
- you don't need to Upgrade your VMs into 16.x VM-version, even if the software is 16.x. Especially this is true if everything works to your satisfaction on 15.x version
- as for a Windows computer = VM, well, you need what you need. That's more difficult to figure out if some KB-update has caused the problem. Anyway, it is a Windows problem. There is no Feature Upgrade Downgrade option - the wording is sometimes like this, but in reality, the way is a new install with NOT doing a new Feature Upgrade (there are settings for NOT doing that in Update Settings). Smaller Updates can be removed per KB number.

As for verifying, which is it, you can make copies from VMs for safe testing and figure out, which of the two possibilities it is. As for Downgrading VM-version of your VM-computer, I think a text editor would do the job. Better to use Google to verify - anyway a test is safe if you use copy-computers.