Although the view is interactive (scrolling vertically seems to be OK) and a cursor appears in the TextField view when it is clicked on, keyboard input is ignored.
The same code works as expected in Xcode 11.5. Simulator in both 11.5 and 12 exhibits expected behaviour.
Is this a bug? I know there is an issue with interactivity in Preview with multiple views inside PreviewProvider but I'd assumed this is if you'd added multiple Views to the same body of this struct.