Before installing a disc brake caliper on a bike, the piston may not return when the swing arm is moved by hand.
This occurs because the piston and shaft have become misaligned due to turning it all the way to the Danger Zone.
If the piston does not return, push the swing arm fixing screw (check the dealer manual) to return the piston and shaft to the correct position.
This will not occur when the product is installed and used on a bike.