Thank you very much, signature-view.
What is the RS-Log telling you?
When you grow from 0% (Sweep), you start with zero points; anything else changes. Sometimes each frame. Motion Blur can't produce anything useful when the points change.
Please explore this file:
Also, if you use the Connect object, if there is animation, the Weld option can sabotage the Motion blur, as it changes the point structure.
All the best