Evolution of the flux-density ratio between the NE and the SW images, measured for each spectral window in our ALMA observations. The error bars are much smaller than the symbol sizes. The flux-ratio evolution based on our jet model (see text) is overlaid for frequencies of 100 GHz (red) and 300 GHz (blue). At high frequencies (e.g., 300 GHz), our model predicts fast and large variations of the flux ratio.