Sure. My system disk was Windows only, but I go to pretty extreme lengths to avoid reinstalling Windows. So after my latest system upgrade I had a UEFI system

Sure. My system disk was Windows only, but I go to pretty extreme lengths to avoid reinstalling Windows. So after my latest system upgrade I had a UEFI system running off a MBR disk and some bad partitioning. This was a source for much amusement...Window Setup would fail without explaining why, mbr2gpt would fail without explaining why, the BCD tools behaved badly, the Windows 10 recovery environment wouldn't work, etc. Gradually, with much trial and error, I managed to work through each problem, until I wound up with a mostly correct GPT disk and a fully functioning system. It's now successfully applied several updates correctly, where previously it would get to 99%, fail, then roll back, only to try again the next day. As a side effect, I now know more about Windows 10 booting, the BCD, and GPT than I ever wanted to. The only "problem" I have now is an unused old boot partition that I'm frankly terrified to try to remove, since it'll renumber all of the partitions and cause some problems. I'm relatively sure I know how to fix it, but it's too risky to contemplate right now.