The only case I am aware of would be using the "2D Pan Zoom" option in 3ds Max viewport while running IR. If you pan farther away from the image center, you will see stronger vignette. This could be described as "no vignette in production render" (the vignette effect is really there, but very weak) and "visible vignette in IR" (because the vignette effect is much stronger farter away from image center).
If it's something else - we will need a scene or a better description to reproduce this.
