Paradox or Riddle. Brain-training exercises.

Water has already been displaced initially; otherwise, buoyancy cannot be ensured. The mass of the displaced water is equal to the mass of the cup with the bolt inside. The volume of the water initially displaced is its mass divided by its density. The volume of the cup walls with the bolt is their total mass divided by their average density, which is greater than the average density of water (otherwise, the entire system would sink). That is, the volume of the cup walls with the bolt is less than the volume of the water initially displaced, regardless of what is protruding. When the system is submerged, it will displace only the volume that it occupies, but this is less than the initial volume.