I don't have experience in Computer Graphics, so I am not sure how the GeometryModifier of the Reality Composer Pro works. But to me there seems to be a bug (Version 1.0 (385.2)).
To test, I created a scene with a flat box. Its center is placed at the origin, i.e. all its coordinates are on one side negative, on the other side positive.
The model uses a custom surface with a grey color, using a MaterialXPreviewSurface node. This node is required to show a preview, in the images below bottom right.
I added a GeometryModifier node to manipulate the Model Position Offset. Its input is thus connected to a Combine3 node that has x, y and z inputs. Without connected inputs, the default value 0 is used.
To make the cube's geometry dependent on the position of a point within the flat box, a Position node is added. Its output vector is decomposed by a Separate3 node to x, y, and z.
If now the x output is connected to the z input of the Combine3 node, the flat box is tilt as expected in the preview, and in the main viewport.
To make the box kinked in the middle, I connected an Absval node the the x output, and connected abs(x) to the z input of the Combine3 node. Now, the preview shows the kinked box, but in the main viewport, the flat box is neither tilt nor kinked, but flat and offset within z.
This seems to me to be a bug, although I am not sure because of the possibly used different coordinate systems, scaling factors, etc.
