I have a project where I use the slice to remove some excess geometry while animation happens to it. But it behaves rather unfortunately.
It will either not render MB or render it wrong. If I increase samples it just adds another object.
If I disable the slice, it looks correct. If I render with CPU, it is fine.
An overall experience, if the samplerate is at anything but 2, it tends to give issues in motion blur.
Here is an example of the scene. It shows the part with MB and no MB
https://www.dropbox.com/s/r3821tbaxfwkjxn/sample\_gpu\_issue.max?dl=1
I found this old post which seems to suggest it is simply not possible to achieve mb with slice. Slice modifier+Motion Blur - V-Ray for 3ds Max :: General - Chaos Forum
Vray clipper seems to work just fine, so that could be your solution here 
The “problem” is that with CPU it works fine.
And the way the geometry samples behave, i dont understand.
It is not in this example, but I have other cases where an object goes to rest and the MB explodes for a frame.
Sorry but I found that with cpu the issue is the same, so not sure what could be the issue otherwise.
Use the VRayClipper as mentioned in the old thread. Motion blur and the Slice modifier do not work well together (changing topology).
