I have been 'successful' in updating to Windows 10 version 1803 with an external GPU (display is connected to the Radeon 580 external GPU). The problem is that this version of Windows shuts off my MacBook Pro's built-in Radeon Pro 560 display driver as it says there is a problem (error 43). I deleted the driver (in the Windows Device Manager), scanned for new hardware, it installed the Microsoft Basic Display Adapter (instead of the Radeon Pro 560 Driver) and this Basic Display Adapter was also stopped due to error 43. If the external GPU is not connected, the laptop screen is black.
To get to 1803 'success' I went through at least 2 combined Windows 10 version 1709 updates (Knowledge Base numbers were only about 3 different from each other) and another driver update.
This would appear to me that, even Microsoft's canned, basic adapters don't work on this, it is not necessarily a bootcamp-specific problem but a Microsoft 1803 problem. Stay at 1709, 1803 is not ready for prime time. If you have upgraded & everything works fine, fine. Otherwise, rollback to 1709.