Windows 10 will boot at startup but not from startup disk utility

I'm going to add to the chorus of Windows 10 external SSD posts, I'm sorry. But my position seems a bit different from most.
I've installed Windows 10 on an external T3 ssd, connected to my 2017 iMac. If I hold down option when I power it up, I can select "EFI Boot" and load Windows 10 and it works fine.

But, annoyingly, I can't select this in Startup Disk utility. A "Bootcamp" Windows drive shows up, but the bless tool won't let me select it. And when I go to disk utility it says there's a problem with the partition map.

The think is, Windows runs totally fine and I can boot it holding down option. So I can ignore these issues, but I'd love to know if there's some way I can fix the partition map or make it work from Startup Disk, which I find a lot more convenient.