Idle current reduction in an X-Controller feature, not a grbl feature. From the documentation:
Idle current reduction is another feature that can be set on the controller board. This setting automatically reduces the amount of current shortly after the motor stops moving, reducing heat from the driver and the motor. This feature is especially great for the Z axis which spends a lot of its time idle. You can set this using the dip switches, just make sure to again check out the diagram for proper placement.
Also check out this Inventables post by @BartDring which says:
The X-Controller idle reduction feature is different. It reduces current rather than stops current. It is also independent for each axis.
That’s what leads me to believe this could be an issue.