Different maths libraries use degrees and others use radians ... well radians is by far more common but in this case the Vuo node uses degrees. Multiple an angle measured in radians by 180/pi converts it to degrees.
I figured it might be something like that. Thanks. I can work around it at the moment by flipping UV and the flipping final image.
And noting that I am confident the warp mesh files are OK since they function as expected in other software that supports that format.
Note that I am not concerned with the rough outline ... I know why that is.
The issue is that by simply flipping the mesh vertically means it stops working.