Hey Avbates,
since on the Onefinity machines, Z home is on the topmost position, you do ➪ stall homing against gravity (➪ see also here). The effect only appears when stall homing. The heavier the weight of the Z carriage and the spindle, the more will stall homing fail. This is because when using stall homing, the controller tries to find out if the axis ceased to move, and it does this by sensing whether the stepper motor produces electro-motive force (i.e. acts as a generator), which it does only when moving. However it can sense this only when there flows no current. But when no current flows, you have no holding torque which means the stepper runs freely, which means, it does no more hold the carriage’s weight against gravity.
You could avoid this by ➪ retrofitting limit sensors instead of using the generally not accurate stall homing. The buildbotics-derived Onefinity and the Buildbotics controllers support the connection of limit sensors.