Sorry I didn’t see your post first: indeed I got my GTX to record when Zwift was in windowed mode (not full screen) using the Windows 10 Game Bar. Here’s the cross thread: Record Zwift Video - Windows 10 - #4 by Ales_Susnik
So, indeed, it’s a Zwift issue, since both the GEForce Exp and W10 GameBar both work in non-full-screen mode.