There is a very slight delay from when you press the control on button the brake for the z axis that holds it from falling until the electronics come on and the servos turn on to hold the Z axis up until complete power up.
This could be one of the green PILZ safety relays.
I do not have circuit prints at the moment to give you a place to start.
But I would look at that circuitry for the Z axis brake.
Regards
DD