Posting for reference should anyone else experience it.
Issue has been sent as Feedback on Insider Program.
The following driver has an issue with RDP access using preview build 1903 (replicated in 18343, 18362.30 & 18875).
Intel Q45/Q43 Express chipset (MS Corp WDDM 1.1) 18.104.22.16802
This is the driver installed by Windows and the most up to date version.
When that driver is installed and enabled, any connecting RDP sessions will login successfully. However, it will only display a black screen with visible mouse pointer.
All of the usual RDP black screen 'tricks' do not resolve it.
Workaround - Disable the driver, then a connected RDP session will display successfully.
The driver works without issue on release build 1809.