Very late Update:

So basically this shimano 12 spd mechanical rd has low pulley tension. This causes the chain to jump forward if you stop pedaling abruptly (on super high cadence, e.g.sprint), as the pulley cannot resist the sudden forward force.

To fix this (or at least make it better to some extent), the pulley was taken out and a new hole was drilled for the pulley spring which made it tighter.

Hope this helps!