DirectX 12 screen flashes

I have an app using the ‘old’ DSPack component to access DirectX.

It has worked for donkeys years without issue, but with DirectX 12 there are a couple of windows ‘Frame only’ windows that flash past when it loads DirectX. It doesn’t happen with DirectX 11, or any preceeding versions.

Anyone come across this before, or got any ideas before I head down the rabbit hole?