There is nothing wrong with your reasoning, but your understanding of the problem is wrong. Who is the index change of the platform scale compared with? It is to compare the reading changes before and after the messenger wire is cut, not the reading changes during the ball falling. Before the messenger wire is cut, all objects on the platform scale are stationary, so the reading is the mass of all objects; After the messenger wire is cut off, the ball falls at a uniform acceleration, and the liquid corresponding to the volume of the ball rises at the same acceleration (this is very important and cannot be ignored). The tray reading can be calculated by the system momentum theorem.

Strictly speaking, because both the ball and the liquid are moving, this requires mathematical deduction to get the result, but there can be a special method for multiple-choice questions. Assuming that the density of the liquid is very small, close to zero, the mass of the ball will not contribute to the pallet reading after the messenger wire is cut, which obviously reduces the reading.