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